Hi 09,
Studying for the GMAT now - far in advance of when you will actually "need" your Score - is a smart choice. Many GMATers spend 3 months (or more) of consistent study time before they hit their 'peak' scores, so starting now - with the goal of taking the GMAT in March of 2021 - gives you plenty of time to study, which is great!
The GMAT Score is valid for 5 years, so you should have no problem applying with an Official Score Report from 2021 for up to 5 years after you take your GMAT.
Since it sounds like you're just beginning your studies, then it would be a good idea to take a FULL-LENGTH practice CAT Test; you can take 2 for free at
www.mba.com (and they come with some additional practice materials). If you want to do a little studying first, so that you can familiarize yourself with the basic content and question types, then that's okay - but you shouldn't wait too long to take that initial CAT. That score will give us a good sense of your natural strengths and weaknesses and will help provide a basis for comparison as you continue to study. A FULL CAT takes about 3.5 hours to complete, so make sure that you've set aside enough time to take it in one sitting. Once you have those scores, you should report back here and we can come up with a study plan.
I'd like to know a bit more about your timeline and goals:
1) What is your goal score?
2) Do you currently have any Schools that you plan to apply to?
GMAT assassins aren't born, they're made,
Rich