How could I pronounce "l" in some words?

Hi stoo, welcome to Englishforums.
It is difficult to explain how to make an L sound. And I also don't know what's your native language and what exactly is your problem, or what variety of English you are learning. Anyway, I tried to find some general material. I hope it helps you at least a little.

http://www.uiowa.edu/~acadtech/phonetics/english/frameset.html <-- All American English sounds, and the position of the tongue and the mouth.

But now that I think of it, since you said you have trouble with "final" L's, then you probably already know how to pronounce an L. Then you need to learn the "dark L". In American English all L's are dark, but they are darker at the end of a word or a syllable. I don't know how I do it, but I think that I pronounce a "normal L" and at the same time raise the back part of my tongue, as if I had to close part of my throat with the back part. And try to say L that way.
Sorry that I can't help more.

Hmm, you are Chinese so... you should have trouble distinguishing R's and L's, right? I don't know how to help you, because in my native language I have both of those sounds (not the dark L anyway).
But I know that if you take a look on youtube, you'll definitely find something for you. Search for something like "L ESL", or "L pronunciation", etc., there are some videos that seems to focus on L and R, and other material. I can't comment on the quality of those videos though, since I haven't watched them.
If you can't say an L alone, and then at the beginning, and then at the end of a syllable (as in LAP, FALL), then you should be able to put it everywhere. Just make sure you try very slowly the first time. Say what you want to say very slowly, and you'll be able to say it. Then try to speed up.