Can you do it for me?
Could you do it for me?
Will you do it for me?
Would you do it for me?
These are all correct. It depends who you are talking to. Starting at the top, it's more familiar and at the bottom it's more formal and polite. If you met the president, you'd use the bottom one, but the first for a friend.
1 If I did this for you, would you do that for me?
2 If I do this for you, will you do that for me?
In 1, you are being cautious- you are not promising to do it, or even saying that there's much chance that you will do it, which is why we use the past and would. You want to check the other person's feelings about this without committing yourself. In 2, the present and will, suggests that you are prepared to do it and hope that the other person will cooperate with you.