Although personal views may vary, the GED or General Education Diploma is counted as an equivalent to the high school diploma. As such, many universities will accept this type of diploma depending on the scores attained.
Universities will often request that applicants have taken the ACT or the SAT. Having high scores on one or both of these tests can often greatly increase the likelihood of being accepted into your chosen university. Many universities also offer scholarships for high scorers on these tests.
Applications to universities often include essays or short sections in which you must write about your reasons for applying. This is yet another chance to show why you deserve the chance to attend the university.