How can I improve my chances of getting into a university?

I would recommend actually meeting with the schools admission department for the school that you are interested in. That way they can tell you what they look for and their requirements. Then you will even get the opportunity to share your story in person of why your high school grades were poor and they can give you recommendations on what you need to do to get into their school. Also, some state schools work with state jr. colleges. For example, I went to a large state school that had a program with the state jr college in which students proved themselves by getting accepted to the state school with the exception that they would complete 1-2 years at the jr college keeping their grades up. It was a way for students who didn't make the grades to basically get a second chance. Also, if your state school doesn't have this you can always go to a state ran jr college on your own. If you can prove that you can succeed in these schools it will look great on your application and will make it easier for you to get into the school you want.