I've started a little over half my tanks with tap water. Use a TDS test to see what it is. If you don't have that, you can use a freshwater nitrate test kit to see where it stands. I believe now that it's not so much the excess nitrates and phosphates in the tap water as all of the extra heavy metals, including copper. Copper, in high enough doses, can kill corals and other invertebrates. In the part of the country that I'm from, most of the tap water in peoples houses come from PVC, PEX, Quest or CPVC water lines, so there is a very low risk of copper. If you use tap, I recommend letting the tap water run out of your water hose for a full 5 minutes before putting any in your tank. This sort of flushes away some of the heavy metals that may have been leached into the water while under pressure from your pipes.
For small tanks under 55g, I would just recommend going to the LFS and buying some saltwater in the salinity that you desire to start your tank. Some LFS have only one salinity, while some nicer LFS have saltwater for fish tanks, and saltwater for reef tanks. Pick what you're going to do and go from there. If you are going to mix your own saltwater, buy RO water, put it all in the tank WITHOUT SAND OR ROCK, add your salt, mix it for 24 hours with a high flow powerhead until completely dissolved, then measure the salinity with a hydrometer OR better yet a refractometer.
I HIGHLY suggest to you that you read the 101 tips thread at the top of the New Hobbyist forum and glance at all of the other threads as well. It will save you time, money, and headache to say the least. Also, buy a good saltwater book to read. The general consensus says to start out with "The Conscientious Marine Aquarist" by Bob Fenner. I highly recommend you start reading and getting aquainted with it before beginning this (hopefully) live long passion and obsession. There is a lot of good information on this site and a lot of helpful people so stick around!