Hiya....
Well, I'd start by looking under the "syllabus" section of the abrsm homepage - and then under theory - that'd give you an idea of what level you are at and any gaps in your knowledge you'd need to fill. The little red book ("First steps") would give you an idea of the content for each exam - it's almost a section per question really (and some questions draw on more than one area). I'd say if there's an area you don't know, then you'd need to suss it out before the exam as it's not a case of hoping it doesn't come up.
The prices for the exams are also on the homepage - I think they're under the exam entry section. You can request a paper exam entry form or submit a request to get a password etc and enter online (the advantage of this is that you can also get your result online, usually a couple of days earlier than you get it through the post). You can enter yourself for the exam and you don't have to take grade 1, then 2 etc, just start at the level you think you're at. You don't get your exam paper returned to you after the exam - but you do get a mark sheet listing the question numbers/topics and what marks you were awarded, out of the marks possible, for each section. You also get a posh looking certificate if you pass

. There's some exam tips at the top of the theory page - entitled something along the lines of "as the next exams approach", which is very helpful, and also a list of theory book recommendations there somewhere.
I knew less than half of the grade 1 syllabus in November and learnt up to grade 3 and took that in February. It was a steep learning curve, but I threw myself into it. I don't think I could have learnt any more of the theory syllabus in 3 months ish. Also, the grades gradually add more creative stuff like composing rhythms and melodies etc as they go up. You'd therefore be hard pressed to learn grades 1-5 and take the grade 5 exams without doing at least one or two or the previous grade exams beforehand (unless your musical knowledge is already good), I reckon.
What sort of level are you at? Are you starting from scratch?
ps posting books out to you either today/tomorrow (second class i'm afraid, as it's cheaper!)
ah... well done Maizie - all the useful links! You beat me to it!