go to (a professional)

To "go to a ___" means to visit a professional to get service from them. Here are some examples:

Have you gone to a doctor?

I think you need to go to a mechanic.

I used to go to this psychic sometimes to get my fortune told.

I think (clause)

In spoken English, you can say "I think ___" before the idea that you're thinking.

I think I'm finished.

I think she's coming.

In formal writing or when you're speaking carefully, you should use "I think that ___" instead:

I think that we need to do a lot more testing before we release it to the public.

a mechanic

A "mechanic" is someone who fixes machines, expecially cars. For mechanics who fix things other than cars, you usually add an extra word before like this:

Did you know she's an airplane mechanic?