Today is the 200th anniversary of the birth of Charles Darwin. Darwin is, of course, known for the theory of evolution. Sadly, this is still dismissed as 'just a theory' by people who don't understand the nature of science, in that everything in science is 'just a theory'. Scientists produce theories which are tested against reality. If the theory fails, and no confounding factors can explain the issue, it is wrong. If the theory passes the test then that test is a confirming instance, but in no branch of science can you say that a theory is 'true'.
Evolution has been tested against reality and has not been falsified. Yes, it's a 'just a theory', but then, so is quantum mechanics, and that is fundamental to the design of the microchips in the very device which you are using to read this text.
Like quantum mechanics, evolution is a theory with great predictive power. It isn't simply a 'just so' story, it helps us to predict and explain what happens if we stop to use antibiotics before we kill off all of the infection - the bacteria that are left were the more resistant ones, and so when they reproduce the average resistance to bacteria is greater. This is one reason why your doctor shouldn't prescribe you with antibiotics if you have a cold (as the cold virus is not susceptible to antibiotics, and using the antibiotics increases the selection pressure on bacteria, thus bringing forward the day when the antibiotic is no longer useful).
Evolution helps to explain phenomena as diverse as extravagant plumage displays, cooperation and self-sacrifice in animals ('good of the species' if not a good interpretation, the basic unit of selection is the genome), eyes, the vestigial organs like male nipples (see link, number 2), orchids that have parts attractive to bees and so on. (Note that discovering that the appendix has some function, for instance, does not disprove evolution, it has removed one of many pieces of evidence. That is all).
Wikipedia has an article on evidence for common descent (i.e. that what are now different species shared a common ancestor) and of evolution - though be aware that these pages could be a target for vandalism from creationists. I can highly recommend The Selfish Gene. It is worth getting a later edition as they contain appendices providing answers to questions readers raised in earlier editions, so if you're ready the book and think 'Ah, what about xxxxx'? There is usually a response. Similarly, The Blind Watchmaker is also a good read.
I do think that these are books which even a fervent Creationist should read - if you are going to argue against something, it is important that you know what you're arguing.