When both of them are followed by verb,they are interchangeable.But if you mean I need a glass of water(noun),then use 需要

“得”indicates the following is something you dont want to do .for example,"我得走了"means i have to hit the road. try to translate "我得去工作了”

需要much more means"need"which depends on you.For example"I need a sandwich."means"我需要一个三明治". 得means"have to",you have to follow it even you don't want to.For example"I have to go to work"我得去工作了。

需要: need 得: have to