How do I decide whether to use 還是/还是 or 或者
Very good question.
还是 is usually used in asking questions. For example:

你想去北京还是上海呢? = Did you want to go to Beijing or Shanghai?
这是橙子还是橘子? = Is this orange or mandarin?

或者 is usually used in declarative sentence. For example:

我想去北京或者上海. = I wanted to go to Beijing or Shanghai.
你可以叫我小王或者老王. = You can call me Xiao Wang or Lao Wang.