I Need help my DR 88' 125 doesnt run very well

:?: Im a kid and dont know much about matenince and repaire, I know how to run and ride my dirtbike that weve had since before I was born in 89 but im pretty sure its a DR 1988 125. I dont know if its an L or not. I can get it started after the 10 times or so but it doesnt idle so i take it out with the choke on and after I rid for a while I slowly ease it off and just keep riding so it doesn't stall. The chain I think needs a link out of it because when I go from slow and start to speed up it jerks forward. The chain has popped off once or maybe twice and I just put it half way on and pushed the bike forward and it rolled all the way back on. It probley needs a tune up and I just know how to ride and start it. I was wonderin whats wrong with it. Or maybe just some advice on how to take care of it or what I could unscrew and clean, "If thats all it needs" to make it run a little better. It doesnt really have allot of miles on it. My dad says a couple hundred or so. Its a four stroke and is yellow and blue and it has four gears. I would appreciate any help you could give me on what to do with it or how I should run it. Thanks :? [/b]