Benwestie,
Welcome, and congrats on learning about the
nitrogen cycle...you are starting out right. Navy scuba's advice is perfect, I will just add a few things.
1. No need to do water changes during the cycle. Just monitor ammonia,
nitrite, and
nitrate (and keep a log so you can see how things progress). When the cycle is done (weeks from now), your nitrate will be sky-high and then you can do a large
water change before you add fish.
2. Yes, top off the water. If is drops a bit, no big deal, but you want enough in there so that your heater is submerged and your fitler works right. Be sure to add dechlorinated water!
3. While it cycles, research the fish you want...visit stores and check them out. Read about all of them...it helps the time pass and makes you even more eager to get them in your tank..
4. Keep coming back with questions. I have found the people on this site are not only great for information, but also for encouragement.
JSK
EDIT - 5. If you can get some gravel or filter media or something from a friend with a mature tank, put it into your filter housing. It can really help jump-start the cycle by giving you some starter bacteria. Avoid products that claim to have the right bacteria, because they don't. Tetra Safe Start (TSS) is the only one that does, and if you decide to use that, read up on it and use only as directed. Since you are already starting your fishless cycle, I would continue with that.