This item order is very good if you focus full power and destruction. Note that it might not be the best choice in some games, such as tanky enemy teams.
The build should be the same in every game up till your boots. Start if doran's ring and go mid. Unless there are better champs in your team who should be fed, like vlad, malzahar or karthus. First recall: get a tome and begginer boots. If you're doing well, you can get 2 tomes or a hextech revolver right away. This will make your soil skill farm minion waves pretty well. whenever the enemy champ is a clean target, aim your dark binding at him. Remember, if your enemy is a mage with full magic damage, the easier it will be for you, since your shield can dissipate mdamage and cc.
Second recall: needlessly large rod and revolver. This will give you sustainabilty in the lane. You shouldn't be mana starved since you have clarity, doran's ring, masteries and runes that give you mana regen. Be smart! You will soon realise if your enemy is hiding from your binding. Don't be wasting mana trying to catch him every time! It has slow missile speed, so they dodge it easily. focus on farming the last 3 minions, and you ought to have a clean aim. Approach and ult him. Beware of flash scared champs. Always keep a ward with you, placing it on side of the river with enemy lanes pushing hard on your team. Those are more likely to gank you.
Third recall: finish our boots and will of ancients. At this point you should be at least lvl 11, with your second point in your ult and a lot of spell vamp. Dont be afraid to take some damage in order to get a clean hit at your enemy. You can always heal yourself with a minion wave or recall and teleport back. Clarity and Teleport are summoner skills aiming for morgana's long duration laning. After you get your boots, you will watch the game for a bit. If your enemies still have no magic resist, get rabadon's. If they have, you should wait a bit more. In games when you're not doing so well, you can grab a rod of ages, to keep yourself alive, and only after this you go for a rabadon's. In any case, after you buy a rabadon's, your damage will be great, so the enemy team WILL raise their mres. Counter it with void staff. Now, your red and blue runes are magic pen. So are your boots. You also have masteries for mPen. And now Void staff. Their team should be careful with you.
Next item is something morganas around the world love buying: Zhonya's Hourglass. This will be great to situations where you have to ult and "suicide" into the enemy team. Zhonya's active will prevent that suicide. If you're the type of player who dont usually remember using itens that have active skills, then you should avoid it. Zhonya's ap and armor are good, but too expensive. It'll might just be in your way. You can always aim for another type of defense, like Abyssal Scepter and its mres. It also reduces some mres of nearby enemies, so, more damage for you! Why don't we have both? We can! But in order for that you have to sell your doran's ring for inventory room and not have a rod of ages, like mentioned earlier.

MAX OUT your tormented soil skill. Enemies don't always step on it, but when they do, they have a hard time for the rest of the fight. It takes their life and their mres!!1!1!!!eleven!! So, before ulting them, place them on tormented soil!
Second skill to max out, dark binding. If you hit them, since you're a mid laner and 2~3 lvls higher than other laners, it will take lots of life from possible gankers. If you can combo, first place tormented soil, then dark binding. In other cases, if you first hit someone with binding, don't forget to torment-soil them right after! And if possible, ult them to death.
At lvl 8 you should get your first point in your shield. It's very good, since it dissipates disables and magic damage, but your other skills have priority, since it doesn't also prevent physical damage, which sux.
Your ult has a long range, and it has a great GREAT stealth detection. Note that it is ONLY avaiable to use when there are enemies nearby. So, when that tricky evelynn appear in your lane stealthied, ult her to death!! Even if you can aim the rest of your skills on her, you should be able to survive a gank and go back to your turret and safety. BTW, I love laning mid against AP teemos. The shield destroy's their shrooms and prevents slow. Plus, whenever he hides with his passive you can detect it with you ult and combo him! Dark binding, Soil and ult. He goes down, without moving and, obviously, without leaving stealth mode.

Clarity and Teleport. This choice is in order to give you early dominance and lane sustainability. If you don't like clarity, flash is a good skill for ya, since most of your enemies will flash away from your ult. Flash in. Normally, they flash away and your ult ends, that's the main reason why I don't pick it. Clairvoyance is good too. Will save you from lots of ganks and will save you some ward money. If you pick it, don't forget to master it! Ignite is ok too. Ghost might be ok too. Exhaust is a no! So is Heal, Fortify, Cleanse, Smite, Rally and Revive! You're not ad, so Rally and Exhaust won't be any good for you. Cleanse and Fortify are for Tankers, which you're not! Smite is for junglers, which is not a good idea for morgana. Revive are for noobs and/or Dominion, but this guide does NOT cover any of dominion's gameplay.

Blue buffs are always welcome!
Morello's evil tome might be a great item for her, but I don't use it since they nerfed her passive spell vamp and will of ancients became a must have. Also, if your team has another mage champ with skills based on healing, such as vlad, fiddle or nunu, you can ask him to buy a will of ancients too! It has an stacking aura, so it is lots of fun in team fights.
If you have trouble landing your Dark Binding skill, you can try to force your target to the area you'll fire it. HOW? Land a tormented soil in the side of the lane. He might dodge to the side, just when you fire the binding. I've done it countless times, but it's risky, since you can't combo him unless you want to approach for ult. He will then damage you heavyly while bound by the skill, and then will run from your ult. This is only good if you're laning with someone else in your team, like bot or top.