how do I get the user id of a token?

Hi everyone. I am beginner, my problem is how to get the information of a user when he login into mobile app. the infomation includes courses which he enrol, his profile... I think to get the information I need to know his Id. but when he login, I just get his token by using API function. so how to get the user Id by user token? or is there any way to get the information of user without id?

please help me! thank so much!