How can I teach students for free for entrance tests to colleges or jobs?

If you sincerely think you have skills that you could help students to qualify then approach the colleges and offer to work as a volunteer.  At that level you possibly would need to complete some formal training yourself to ensure you have some basic teaching skills though.  

When I could not find suitable part time work I used to volunteer at my son’s primary school a few hours a week, mostly to help students with their reading and comprehension.  I was quite surprised that so many children were struggling, as my own son was reading of his own volition when he was three.  It was very rewarding to hear most children improve and their confidence grow.  Background checks were required (today they are called “Working with children” in Australia) and I also completed a course, run by a speech specialist, to help children with language difficulties.  I am sure you may find some at college level that also need help with some basic language and comprehension skills too.  

There are probably bulletin boards for your town.  Post an ad there with your academic credentials.

Just ask around and find out whom needs your help.  

Pray and ask God for whom you can help here.