What topics do you need to know for Grade I?
This is a link to the syllabus where you can see what you're tested on:
^ Nope, can't do link - the system insists on changing the url to "forums.abrsm.org/" so go to "Our Exams > Our Syllabuses" in the menu bar.
The ABRSM's First Steps in Music Theory pocket book is divided into sections for each grade which makes it very easy to follow, and the step-by-step format makes the information easily digestible.
When will the results come back?
From what I can remember, the results take about 6 weeks, although that timescale may have changed since I took theory exams a few years ago.
Where will the exam be held?
I believe the exams are usually held at a local school, although "local" may depend on where you live. If you live in the middle of nowhere, you may have to travel to the nearest large town.
I was recommended 'Pass Grade 5 Theory' by Dorothy Dingle which takes you all the way up to Grade 5 in one book. It is a text book and workbook all in one with tips and hints in passing the exam plus a separate answer book.
I've got the first edition of Dorothy Dingle's book and I liked it a lot as it was very clear, user-friendly and comprehensive. I can't speak for the 2nd edition, but there were a few errors in the first edition which hopefully have been addressed in the 2nd edition.