A few things to take into consideration when deciding which watch to buy are, what will you be using it for and what your budget is. I've basically put watches into 5 main categories of style.
Casual Watches: These are for your everyday wear. When picking this style think about what colours you wear the most. Consider a comfortable band such as leather or maybe rubber for light weight. Make sure it is easy to read and should have a date window displayed as well.
Dress Watches: Personally I like a dress watch to stand out, so I like them a little bigger then normal, and accented with diamonds around the bezel to help (this is not usually for everyday use). A rose gold or white gold look are my ideal colours.
Sport Watches: When you need a watch for function such as stop watches, alarms, and timers you'd probably want to consider a digital watch with water resistance, or if analog is the look you want make sure it has a rotating bezel timer to suit your needs and a steel or rubber band for water resistance as well.
Fashion Watches: When it comes to fashion watches all rules go out the window. Even though you can't really just wear any clothes you want to these days (even though some do) when it comes to your watch put on what ever you want, maybe a chunky watch, one incorporated into a bracelet or some neon colours. The choice us yours.
Luxury Watches: These are the finest hand crafted money can buy. Not for people on a small budget, but for those who want the finer things in life. These watches are usually hand made master pieces with precision timing, produced in smaller numbers and thousands of dollars. There are many companies making luxury watches but the Swiss are definitely world renown for their designs.