Mercury is hard to see as it never ventures very far from the Sun. Due to its elliptical orbit, its maximum separation from the Sun ranges from about 18 degrees to 28 degrees (depending on whether it is near or far from the Sun at that point).
The best time to see Mercury in the evening is near the spring equinox (for observers in the northern hemisphere). Near the spring equinox, the ecliptic points almost straight up after sunset. Therfore, if Mercury is 18 degrees from the Sun, it is also 18 degrees above the horizon (if the ecliptic is at a sloped angle, Mercury could be 18 degrees from the Sun but substantially less above the horizon).
Tonight Mercury was as far away from the Sun as it gets this spring (a relatively poor 18 degrees as Mercury is about as close as it gets to the Sun right now). That is still good enough to get Mercury well above the horizon a half hour after sunset or so. Here is my pic from tonight. Mercury is to the right of the tree.
If you want to see Mercury, don't dally. It will be fairly good the next few nights before it starts a rapid dive toward the Sun. There is not much else out there bright right now, so it should be easy to pic out.
Reprinted with permission from the Half-Astrophysicist Blog.