when to use el/ella/ud
"él" means "he"
"ella" means "she"
"usted" means "you" in a formal sense, like if you were talking to your boss. Another example is that my spanish friend has to use "usted" with all his teachers.
"tú" means "you" in an informal sense. You would use it when talking to family and friends.
How/when to use them
Normally, these aren't used unless you want to clarify/emphasize who is doing it.
"No, tú vas a pagar." means "No, you are going to pay."
Normally, you just leave them off.
"(yo) Quiero jugar." means "I want to play."
"(él/ella/usted) Quiere jugar." means "He/She/You(formal) wants to play."
"(tú) Quieres jugar." means "You(informal) want to play."
I put parentheses to show where the "he", "she" and "you" would go if we chose to use them. Please tell me if I screwed up, or if you have any more questions.
BTW: This post should be in the Spanish section of the discussion, you would get more help there. You can change it now by clicking edit on your post and changing the section to "Spanish".