I can tell you a very easy way to distinguish the two words:
成立: a verb for some actions
建立: a verb for some ideas or some thoughts
成立学校 成立研究所
建立关系 建立友谊
by the way, there is another word: 组建 when some parts unite together and become an entirety, we use this verb
for example, 组建政府 (政府: governement, it is made of several parts.)