I run a small framing business (that is... a small business making frames, not a business making small frames...
) and for what it's worth, here's my thoughts...
Good quality framing ain't cheap. You gets what you pays for, and I'm sure we've all seen some shocking quality framing. If you're not doing it yourself, find someone with a good reputation who you can trust, and ask them questions. If they're good, they'll be able to offer invaluable advice.
Simple black frames are a classic look and well suited to photographs, but don't discount other styles. There's some really nice mouldings on the market that will present your pictures beautifully. If it's a reasonable sized frame (and 18" sq print plus mount width is getting on for "reasonable" size), don't go for a really thin moulding, they can tend to twist along the length. With the weight of the glass, a frame can be surprisingly heavy, and a wider / deeper moulding will make a stronger frame.
On the subject of glass, there's loads of different types, ranging from cheap and cheerful to heart-stoppingly expensive. Most framers should have some samples that can be laid over your prints to show the clarity. Standard glass has a green tint which may alter the tone of your pictures. Waterwhite glass doesn't have this but is more expensive.
I'd echo zapsnaps with regards to white mountboard. A look at any mountboard catalogue will show you probably a dozen or so variations on the "white" theme... I'd look towards an antique white, which is softer and a little more neutral in tone than a "bright white". Once the colour/shade is decided, the main consideration is quality. Low quality board will turn yellow / brown on the cut bevel after a while, so go for conservation grade board at the very minimum, or museum grade if you want to spend a little more.
Apologies if I've rambled on a bit, but I hope I've given you some useful info.