Those two sites in the above posts nailed it. Everything available shows up at those two, one way or another. Both have extensive links to further info, sometimes at the specific agencies/departments involved. For the military stuff, the .mil extension will get you anything you need, as in Army.mil, Navy.mil, etc. At the OPM site, most jobs are listed twice. One posting is for current .gov employees, the other is for non-.gov employees. You have to read them both to see which one you are looking at, but this info will be on the first page you see.
As a new employee, you could expect to come in at the lower end of the pay scales shown, without some experience or an advanced degree. The more experience you have, the more likely you could get more money. Over-qualification is an issue, so don't go down the scale too far in that respect. It won't help, and can actually queer the deal, in many cases.
Hope this helps a bit.
Added: The military sites also have civilian jobs as well, but those are usually at the OPM site.