You take a test depending on the type of job you're interested in. I'm in the Washington, D.C. area and use to work at the Pentagon for the Navy for a couple years in the mid-1970's.

If you're a college grad you take the professional test (most competitive, I took it right after college but it was a recession like now (1972) so all the college grads were applying and the Vietnam war was still going and vets get a 5-point automatic advantage) and there's a separate clerical test. There was no way I could get a professional job in the area; I didn't have the background skills (there's a wicked long background form you have to fill out too, with every detail of what you've done all your life, all your skills, foreign languages spoken, etc. The professional test has to test for "all" professional jobs so the background papers have to get "all" background too) and the competition was so fierce you had to get in the high 90's on the test to even be considered (since the Vets could technically get 105 points). I think I scored in the 80's. But the competition is probably not as bad if you're not on the East Coast.

Depending on your score (professional start at GS7-9, clerical are usually below that, you get a ranking (I started as a GS3 and then was promoted to GS4 after a year; it's based on a combination of salary and seniority) and either agencies in the area looking for help of that rank call you to try to get you to interview with them or you can apply for a specific job of that rank. I didn't know the agencies would call you as I had a family friend who knew of a specific job I could apply for/get in his "department" but first I had to take/pass the clerical test (basic English/math/general ed written as well as typing parts and there were other, more advanced tests at the time, shorthand, etc.). I was shocked when I got phone calls from 3 other agencies trying to recruit me so I could have chosen :-) but since I was just taking the test as a formality to qualify for the job that was available in the friend's office (and during the time when there were more clerical jobs available than workers applying instead of the other way around like now) I just did that.

The job I got technically needed shorthand which I didn't have but they mostly needed someone with half a brain (it was working with Navy JAG lawyers) and once I was there, they made me take a Navy shorthand course (which I hated and didn't do well with but was kind of fun as it got me out of my office an hour or two a day for a few weeks :-) so I'd meet all the requirements of the job. One of my bosses dictated quite a lot but was a Lieutenant only a couple years older than I was and reminded me of my brother, also a Navy lieutenant but not a lawyer, so I had no trouble sassing him :-) We got along fine but my shorthand was so slow and weird he "learned" it from my taking notes, from upside down and use to correct me! LOL

The salary was nearly equal to what I could get in a company job but the job setup is much more rigid/boring so I couldn't stay there all my life! Being Government, there was a certain "grayness" :-) to the jobs, they were all "alike" and I found it hard to be very enthusiastic or care after awhile. Think of all the State or Federal agencies you deal with now; that's what working for the Government looks like :-) It's mostly paper pushing bureaucrats, there's little originality or creativity at all.