Welcome to the forum Sam.
Have you kept some other kinds of fish before? If you have we may be able to easily guide you to the differences between them and tropicals. Often we find the differences are minor.
Unless you have severe water concerns with your tap water, it is often just fine as fish water once it has been dechlorinated.
Something Nurglespuss said may be a bit misleading. You cannot use straight RO water and expect any common fish to survive long in a tank. You can mix RO with tap water to acquire a suitable blend or you can "reconstitute" RO water into fish water by adding chemicals to the purified RO water. That is much like what they do for many forms of bottled drinking water. All chemicals are removed and a mix of salts the manufacturers believe is tasty is added back in before it is bottled.
You will be embarking on a filter cycling adventure once the tank is set up, there is no way around it. There are options though. You can choose to cycle the tank before adding any fish, which is far less work. You can also cycle the filter with some fish in the tank, which can cause a new fish keeper a good deal of stress while trying to keep his fish alive by doing daily large water changes on the tank for the first couple of months. The approach you take should be thought through before you get any fish at all. I have a link to instructions for both types of cycle in my signature area.