You need to choose a field of study, then find out what universities have a good reputation in that area. In some locations you will be able to enter a university without a concentration, but that usually means you waste a year of your time while you work it out.

Each university will have different requirements; and within each university, the requirements by subject specialization (or ‘major’ in the USA) will vary.

It depends on what country you live in. In some countries, university admissions to all universities are extremely competitive, and unless you do very well on certain exams, there won’t be a place for you. In other countries, like the US, there are universities and colleges at all sorts of levels, from the best in the world, to ones that are much less competive. In the US, even with a so-so GPA and test scores, there is probably a place at some university for you, somewhere. Not at the most competitive unis, no, but somewhere. And kids who do quite poorly can actually go to a community college for two years, raise their grades, and then apply to a full university.

Roaringmice is right, it depends entirely on your location and on the university you are trying to get into. I work at an open-enrollment state university in the U.S. and we take virtually anyone that has a high school diploma or GED. However many of these students don’t finish because they weren’t prepared/didn’t take it seriously/other circumstances. Just because they allow almost anyone in doesn’t mean that college is easy, it just means they give you the opportunity to prove yourself.

It depends on where you and what schools you seek to get into. For schools like Harvard, Stanford, Michigan, Berkeley, the GPA requirement is probably higher than 3.5.

Many smaller, less famous universities will let people in with decent scores.