Kog maw is best for playing mid!
Based on your opponents play style you decide how to act. If your opponent isn't harassing you, then it's good to farm until lvl 6 ( Or if fb isnt taken, by lvl 3 you should be able to take him/her down, unless he/she has exhaust). As soon as you reach 6 start spamming your ulti after every 4 secs. That way it costs only 40 mana and if you land your shots, you are keeping your opponent away from farm or engage with flash + W skill and get a easy kill.
And if you have aggressive opponent, go all out W skill harass, You will be amazed by the damage it deals.
Oh and ofcourse, if you land your shots good and send your opponent to base, go and gank other lanes.
Mid game should be easy if you get some kills and good farm. Mana buff is something you really need. Good for scouting and harassing enemys with ur ult before the figth even starts!
This goes for Late game And Mid game. Let your tanks go in firsth, hold some distance and get theyr carrys down from sides. You can get them seperated easily with your ultimate aswell. So basically, you deal 10 % of theyr hp into damage so you are good for taking down tanks aswell. alot of people dont understand that but it may be nessecary sometimes.

So, i prefer taking Dorans Blade as my 1st item because it gives you some health. And you can always sell it if you are missing some gold from your main item.
If you are doing well it is useful to buy early Madreds Razor, even before boots. That way you can make sure your farm is on top. With your attack speed runes creep killing should be piece of cake. After that you can buy berserker greaves or just boots of speed with rescurve bow. Rescurve is good item for snatching some early kills plus extra speed farming.
If you get Madreds bloodrazor, getting kills goes easyer. This passive plus your W skill does epic damage early game.
Zeal comes now because of that little bit speed you get. It helps u escape ganks and so on.
You all know what other items are for :).
If theyr carrys dont buy armor or magic resist, with infinity edge you do like 500-700 crits.
And ofcourse, buy some defense items. at least one. banshees vail or thornmail. I didn't include this because you need to look who does max damage to you and get some def for that.