This is something you probably hear a lot from Creationists: "Evolution isn't true because it's just a theory, even scientists don't know if it's fact." This is of course the result of a bad definition of what a scientific theory is. Without re-writing the entire page in the link with this post, a theory in science is the explanation that ties a group of facts together. A theory must be falsifiable, and it is something so reliant on well-known facts that it is widely accepted as truth. However theories do have a tendency to change, in very subtle and little bits at a time, so that the main idea remains intact but a few small tidbits might change based on new evidence. It's like when a person changes their socks, the different socks don't really affect how the person functions at all, we might not even notice the change without examining very carefully.
The definition of a theory in science always seems to baffle creationists and the like because they're trying to use the everyday definition of the word theory to disprove a scientific explanation for a set of facts. The theory of gravity explains why gravity happens and how it works, as does the theory of relativity. Any scientist will tell you that these theories have been proven true too many times to count. The theory of evolution is no different, we have plenty of evidence and facts to back it up and it can also be observed in such things as the evolution of HIV. So don't let creationists try to tell you evolution is just a guess and we don't really know how we got here, because we do know that evolution did happen, there's really no question about whether it happened or not we just continue to look at the other questions surrounding biological history, How? Why? When? Where? Etc.