'She's thinking about studying Law' = She isn't studying Law at the moment but is considering the possibility of enrolling on a course. This is present continuous used to talk about a process that's happening around the present moment. Further examples: I'm thinking about going for lunch, I'm hungry. She's thinking about quitting her job, she hates her boss!
'She (adverb of time e.g. often/never) thinks about studying law' describes a general behaviour that the person does often. This is the present simple, which we use to talk about routine, so we'd usually have an adverb of time in there too to indicate frequency. Further examples: She thinks about going on holiday from time to time, but can't really afford it. I think about calling Alex sometimes, but he never picks up the phone.
Hope this is a bit clearer for you!