Your first question was pretty much answered as far as adding dechlorinator to the water. I agree with testing your tap water to see the levels of ammonia and nitrates so you'll know up front where you stand.
Since it's your very first water change, and I'm assuming you're doing it right after your cycle ended, then 50% is not too large. You can even go larger if you'd like. After adding inhabitants however (and this varies in opinion!), I would do about 10-25% weekly. The longer you choose between water changes the more water I would take out.