First of all, interfaith marriages can work out quite splendidly. I’m a seriously practicing cradle Catholic and my late wife was a seriously practicing Zen Buddhist, and our marriage was sheer bliss for both of us. What is needed is absolute and total respect for each other and each other’s beliefs. If this is not the case, or if you are entering into the marriage with even the slightest hint of a trace of expectation that you fiance is going to convert some time in the future, then it is probably not going to be as blissful. If the latter is the case, then I agree with the other who said it would be better for you to marry someone who shares your faith rather than betting the farm on the off-chance that your current fiance is going to convert. Best to cut your losses now than wasting valuable time on a relationship that has little, if any, chance of working out.
Second of all, when discussing our faith with non-believers, they often ask, sometimes totally sincerely, about the unsavory events in Church history, of which there is no shortage. The automatic response is either to deny them, or whitewash them, and either of those responses are likely to turn them off. All religions have inspired great people to do great things, and all religions have inspired horrible people to do horrible things. Catholicism included. We have a lot of skeletons in our closet, and trying to dismiss them or paint them in a better light is not the way to earn respect for the religion. Learn about these episodes in Church history, and how the Church learned from it’s mistakes. The rejection of antisemitism with Vatican II is a good example. Avoid any apologetics books that tend to gloss over the dark side of Catholic history or explain it away with cute excuses. They are unlikely to convince anyone else, and might even undermine your own faith.
Third of all, there are different “levels” of religion, from folk religion that borders on superstition, to legalistic religion, to philosophical religion, to mysticism. The first two are what non-believers usually have the biggest problems with, especially if they are well-educated. And, frankly, so do a good many Catholics. On the other hand, they are often fascinated by the latter two. Learn about the rich history of Catholic philosophy and mysticism. But also learn about folk expressions and the legalistic mindset, so that you can appreciate how outsiders often view our faith.