"Would" is usually for something hypothetical, a lot of the time used with "if." "If I were rich, I would buy another car." Or, "What would you do if that happened?"
"Will" is the word we use for future. "I will go to my friend's house tomorrow."
This is a little more complicated, but you can also use "would" for past future, as in, if you are telling a story in the past and want to talk about something that has not happened yet in the story. As in, "I knew he would write me a letter."