Depending on where you are located, not every type of exam is available. You can find the nearest place to take your A1 exam via the venue finders below:
As soon as you feel ready. Keep in mind that certain exam centers only offer exams a few times per year or that in hot spots like e.g. Berlin you might have to wait 6-8 weeks or more before you even get a spot in an exam. You can check that out already now with help of the venue finders above.
If you are uncertain whether or not you'll pass your A1 exam and time is of the essence, a good strategy might be to register for 2 exams at different institutions right away. Leave a week or two between them if possible so that you can practice those things that you realized need practice when you took the first exam. In general that's not necessary but I've had a client that even registered for 5 exams (on B2 level) because their livelihood and honor was at stake. He passed the 4th exam but failed the other four.