I agree with the answer of Cherry! I'm going to give you some examples, and I hope those will help you to understand more the difference between "juede", "renwei" and "yiwei"
"我觉得很幸福。wo juede hen xingfu" I feel very happy
"我觉得她的衣服很好看。wo juede ta de yifu hen hao kan" I feel that her clothes is very beautiful.
"她觉得不太舒服。ta juede bu tai shufu" She feel not very well.
"我认为他可以成功。wo renwei ta ke yi chenggong" I think he will be successful.
"我认为你应该打电话给他(给他打电话)。wo renwei ni yinggai da dianhua gei ta (gei ta da dianhua)" I think you should call him.
"我认为你可以做得更好wo renwei ni ke yi zuo de geng hao!" I'm sure that you can do better!
"我以为你会来看我。wo yiwei ni hui lai kan wo" I think that you want to come to see me. (In fact, you didn't come)
"他以为我喜欢他。ta yiwei wo xihuan ta" He think I love him. (In fact, not really)
"我以为他已经出去了,原来他还在房间里wo yiwei ta yijing chuqu le, yuanlai ta hai zai fangjian li" I think he went out, actually he is still in the room.
You can use "juede" to show your felling
"renwei" for the things you have made sure, or the things in the future
"yiwei" for the things that opposite with your opinion or in fact