If somebody does something for you, they are usually helping you. It is used more in a positive sense. "He carried my bags for me." "She repaired my bike for me." If somebody or something does something to you, it affects you - it can be either good, bad or neutral, but more often bad. "That last beer did something to me. I think I'm drunk." Also, if something affects your feelings, it does something to you. "That movie really meant something to me." A person can also give something to you (not for you). He sent a package to me. BUT He bought a gift for me. These are just some of the uses, but they are some of the main uses. Look the words up in the dictionary. Sometimes, dictionaries don't help, because they don't give as much context, but it is a good place to start.