Hello, I've been playing Diana quite frequently as of late, and so far I've only lost rarely, even while going ranked. Well, it has boosted my confidence enough that I've decided to share the build I use with everyone else :D (as I don't use Mobafire, my build doesn't tend to match everyone else). Well anyway, enjoy the guide. Also, as this is my 1st Mobafire guide, feedback is always nice :D. I'm doing most of these chapters by browsing how other people make them :P but of course, I don't know how to make things as fancy as them.

Short Cooldown's (thanks to Nashor's Tooth, her skills are rather spamable, even her passive, and by late game her Q only has 2 second cd)
Good Sustain (since her attack speed allows her passive to carry her past being oom (out of mana))
High Burst (With all the cooldown reduction, she can quickly burst down one opponent and hurry to the next, and her passive skill will easily take down whoever survives the initial burst)
Strong Teamfight Power (thanks to her passive once again, she is allowed to not only deal damage to her target, but all around her, that added to all her skills being aoe (area of effect) means she hurts everyone)
Global Aggro (Personally, using this particular build has led me to easy early kills, as such, you'll have a lot of attention of the other lanes. This means that the other lanes can push easier if you can manage to survive ;D)

Mana Starved!!!! (although Nashor's Tooth allows her to go beyond being oom, her low cd's easily convince's a person to spam her skills, meaning you run out of mana rather fast. To counter this, remember to ask for Blue alot ;D)
Squishy (unlike other possible builds, this build doesn't offer very much resistance to attacks until later in the game. So it is absolutely vital to pick your fights carefully)
Global Aggro (With this build, early and mid game is a breeze, and you can easily take on 2 people at once. Problem with that is, you attract a lot of attention from the other lanes, gata watch your back ;D)

Summoner spells :D! Well personally, I never liked using Ignite, I know it's extremely effective but i think it ruins the fun of chasing the kill (I know I know, it's a stupid idea :P). I use Flash for obvious reasons, as it gives me the mobility I need to escape (or chase ;D) as the situation arises. As for Teleport, i enjoy having it to help Backdoor or prevent Backdoors (smart right? :D). I also have Teleport because it allows me to help out an ally thats being ganked. Overall, Teleport is my favorite spell.

Good Alternate Spells:
Heal: I love heal, it's gata be my 2nd favorite spell hands down. Most people hate on it, but thats because they don't see the value of it in a teamfight, or even while your laning. Heal offers a quick beef up during laning to bait people into reckless moves, or even save partners who run to you for help. If it's even timed right, it beats ignite's early laning power.
Ghost: I've never been a fan of this spell, but I see the obvious effectiveness in chasing down a kill in the same fashion that Flash does.
Ignite: Although I don't like ignite because I think it ruins the fun, that doesn't down it's effectiveness for people that don't mind.

Bad Alternate Spells:
Clairvoyance: It's a supports spell :I no need for someone people are gana expect to carry.
Exhaust: Same as above. In addition, Diana will have enough burst with this build that without a doubt, the person you fight, an't gana get out.
Revive: I would thoroughly enjoy this spell, but it's long cooldown makes it almost ineffective.

Ahhhh items, the moment you have all been waiting for. I'm sure you all have question's so let me explain piece by piece:
Early game you will have a ton of harass on the enemy with your Q, and your W allows you to tank your enemies counter harass. Only problem is...MANA. Mana, mana, mana, you can't live without it (yet ;D) so i always start with Doran's Ring. With your easy harass means you can dominate your lane and have all the farm to yourself, so Doran's Ring allows you that extra sustain you need.
Of course, as a melee champion, your definetly going to need boots to catch up to people (at least until your lvl 8 ;D). By the time I go back to base for the 1st time, I love to 1st purchase boots, then another Doran's Ring for the extra sustain, and then Red Potions as the situation demands. With all these simple items, you shouldn't be scared of your lack of sustain.
And now, i'm sure you all have question's about why i choose to get Nashor's Tooth, or better yet, why so early. Well, Nashor's Tooth is what gives you sustain past oom, as long as you've been keeping your Doran's Rings ;P. Also, because of your Passive, it means that your able to easily harass the enemy jungler when they try and gank, or burst down anyone that tried to duel you :P. Also, again, because of your passive, Nashor's Tooth helps you take down turrets in a breeze, as soon as a lane is open, there is a turret going down ;D. Most important I DO NOT recommend you to build Rabadon's. I realize that I have placed Blasting Wand and Needlessly Large Rod next to each other (the prerequisite's of Rabadon's) BUT you want to get Rod of Ages before Rabadon's Deathcap for the sustain, for reason's I shall explain in the next chapter :P.

Now for me, Mid game is when you reach between levels 8-11, at least when I play Diana. By now you should have some pretty decent damage, because by the time you have your ulti, it's feeding time ;D. Now, mid game, you aren't going to be in your lane very much. Unless your jungler is insanely good and all the lanes are having a breeze, mid game you begin to roam. A vital point to help prevent you falling off by Late Game is to get fed. For this reason, mid game you begin to run around to the other lanes and smash the **** outa the enemies ;D. This is why I recommend getting Rod of Ages before you try and make Rabadon's Deathcap, for the extra health. The best part is, if you managed to get fed easily enough, you should have enough damage and beef to be able to dive enemy champions. Although use this at your own discretion, you can dive in after champions after you get your ultimate, but this is not an excuse to begin charging at enemies that are full health still. Friendly tip for the diver's: remember to have moonlight on your enemy before you try and ulti ;D.

By the time you are reaching the end of your build, your role in the game changes slightly. Early and Mid game, you are easily able to take on multiple champions with your short cooldowns, but as we start reaching late game, this advantage begins to fall off. If you've been fed a lot, it's still possible to take on 2 champions at once, but otherwise you aren't going to be a one woman army anymore. Your role in late game is split into 2:
1) pick off the easy targets: dispite not being a one woman army, your burst is still something to be feared, try and pick off enemies careless enough to stray from any help.
2) CHAAAAAAAAAAAARGE: Show no mercy! in addition to the above, your team is going to rely on you to start the team fights, meaning, your gana be the 1st one in, last one out. try and stack up on the tip i gave you before. Hit a squishy enemy like the adc (attack damage carry) with your Q, and then follow up with a hard Ultimate burst.
don't mistake this as saying that you'll be the tank for the team, leave that role to the experts :P but it is extremely vital for you to take out the high damage targets for your team so that you can win the fights.

I realize that the builds between the jungle and laner are rather similar, the major difference is when you begin to gank. Obviously, it's the jungler's job to gank. Now this is another part where I will be raising the "Wdf?! what kinda noob are you?!" kinda questions:
I will be explaining how to jungle from a perspective on going without a leash. I hate leash's, and here are my reasons as to why: There is the high chance of your exp getting stolen, you run the risk of losing a 'leashed' buff, and it steals your allies away from their exp and their farm. overall, i hate it. And, luckily, Diana is among the few champs capable of getting a buff by herself ;D.
Now, you ALWAYS ALWAYS start with W. Why? because it does the same amount of damage as your Q, has about the same length of cd as Q, AND give you a shield (or 2) to take damage. Your W is your best friend while early jungling.
Start with the Wolves, as they are conveniently close to the Blue Golem, and spawn before him. By the time you finish wolves, it's about time for the Blue Golem to spawn, be sure that your W is not on CD when you choose to engage him, as it's going to save your life ;P. friendly tip: hit the Golem/Alpha Wolf before you activate your W, that way you make full use of you W. After getting blue, your going to have your Q, making jungling fast and easy. Q into the area the creeper's are in, and then charge at them with your W.
For anyone new to jungling but would like to try jungling, the typical cycle I run before going back the 1st time is as follows: Wolves, Blue Golem, Wraiths, Baby Golem, Wolves, then Wraiths. If your smart about it, you should be able to do all them without any problems, and by the end, you should be level 4. This is when you take your 1st trip back. (pssst...now is when you start ganking.)
As a friendly tip to all fellow junglers, or people that would like to learn jungling, REMEMBER TO PING. Or ask for ping's. I know when I started jungling, my map awareness was horrible (meaning I never knew what was happening outside what was happening outside my own screen). To help with this, make sure to have your friends ping when they need help, the loud noise is my sign to help someone out. And Vise-Versa to all you junglers, don't just expect your laning allies to know when you are coming, or when you are engaging. Remember to ping before you run in, and ping the person you are going to target 1st. This allows your teamates to have a heads up, and help in focusing on single targets to bring people down.