Diana is a very very strong champ, but weak in early game, so i fed some times with her going bot or solo top, i prefer jungling her, or some times go mid, nice counter to all melee ap, because doing tons of damage at a nice range :D
But so in Jungling, her ganks can Bring you, or your mates to legendary easy....
Hae fun with playing her ;) :)

The Pros:
that damage O.o gigatons :D
easy jungler
strong ganks
not a bit squeechie in mid/late game
beautifull assassin :D
fast jungler

The Cons:
no gapcloser pre lvl 6 (enemiy can easy escape ganks)
no good selfescape
very blue dependent, farming/ganking takes much mana
counterable with much cc

I take no armor sealsbecause you have a fat shield if you have enough ap.
For glyphs i use late game ap-runes for more damage in late game.
Quints of Force are very usefull for mid/late game damage.
I use magic pen marks to bring my damage more efficient to enemies.
In the glass cannon build, I take attack speed marks and seals.
These runes bring a lot of damage you can deal with, also it brings your shield to maximum, so you have a really strong protection.

For Ap-jungling i choose 21-9-0, the standart way, because you need the damage masteries, aswell as the defensive masteries for jungling and because your hp are very low at the beginning of the battle. Summoner´s Resolve is nice for the ten extra gold every 60 secs, because this build is expensive, i take it every time in jungling with me.

I choose Smite for jungling, because of the usefull help to kill the blue and red in early game, late game good for dragon and baron.
Diana is a nice assassin, but have a big weakness, she have no good escape, so i just use Flash to escape, or jump faster into a teamfight, or to finish a enemy who flashs himself away.
if you don´t want to take Flash, exhaust is a good alternative for perfect ganks.

Just easy to say... the q deals giga-tons of damage, i did a penta in my first game with the q-r-q-r combi so just level the q on level 2 and finish it as soon as possible.
To have more defensives, start with your shield , the w and get it again on level three, than finish it secondly.
the e is usfull to gank, because your enemies can´t escape that easy from you and your mates, it´s dealing no special damage, just slowing a bit more, so finish it at last.
Your r is a Skill with the highest range i ever saw (more than Varus long range q...) so perfect to gank, in combination with your q it have no cooldown, with this build, q cooldown 2.8 secs, so do 1 and than r, again q and again r andd every carry should be death, if alone.... a bit op, but ok :D

The start for her should be boots and heal-pots boots getting updated not that late and pots must be because you´ll spend some life during jungling.
Doran´s ring is usefull, because your skills cost much mana and you don´t have much, also it is giving extra hp.
Hextech REvolver for spellvamp, and later the Gunblade is nice.
The codex and nashor´s tooth are a perfect item for Diana, cooldown, mana regen, attackspeed for her strong passive and more ap.
Sheen is cool, for a bit more mana and the extra damage for the auto hits, pre item for Lich Bane later(passive, lich bane= about 1200 damge in one auto attack O.o).
Rabadons is to bring you as much a as possible, every ap should buy it.
The Gunblade will let you survive much with extra ap,ad and spellvamp+lifesteal(remind, you´re melee)
After this finish Lich bane, your Passive will be a nuke :D
Rylais is the gg for the enemy team, it brings you a fat slow, more ap and allover more hp, so you can tank more ****, and beat the enemies up like a boss :D