The kind of champs I like to play and am best at are ranged champs like ryze, swain, ashe, corki, teemo... so i like that kind of playstyle. but my problem is that I totally own early game and get ahead with kills and stuff, and then late game I totally get destroyed. This happens in maybe 50 percent of my games. the other half i do just as well late game though. what is up?

If you "totally own" early and you're losing late, I can really only think of two causes:

1 - Your build could use some work. Perhaps you're not building enough Magic/Armor Penetration?

2 - Your teamfight skills need help. Perhaps you're positioning yourself badly, going in too early, staying too long, getting ganked alone when you should be glued to the tank, etc.

If you think either of these sounds familiar, let us know and we can try to help further.

maybe its positioning...

Build to strengthen you and counter your opposing team, you can't be a glass cannon all the time.

If you're dominating as Swain early game, you should be able to 1v1 pretty much anyone on their team, often staying alive 1v2 or 1v3 until the rest of your team gets there to help. Spirit Visage is a great item on him, as are other defensive AP items (Zhonya's, Rylai's, Abyssal). You don't have to build all of these, but throwing one or two into it based on the enemy team could really help you out. If you're not getting Golem often, I would probably grab a Chalice (oh, and mana runes are a must on him, get scaling mp5 seals); this lets you keep your ultimate up for absurd amounts of time, while still casting your other spells.

Work on your last hitting, if you're harassing well, or even out killing the enemy team, but they're getting more minion kills than you, something is wrong.