Good for you! I did Grade 1 tuba for charity three years ago, and am now entered for Grade 5
Getting to grips with a string instrument all by yourself is a formidable task. If you're not able to have regular lessons, then do at least try to find someone who can help you get some of the basics in place. The trouble is that you have to get so many things in place (holding the instrument, the bow hold, the bowing action, etc.) before you can make any decent sound, and it's hard to self-diagnose which of them may not be working correctly.
Have you got an instrument yet? Unlike violins, there's no fixed 'full size' viola, there's a variety. This is because the ideal size, in terms of acoustic properties, is too big for most people to play. The measurement used is the length of the body, in inches: 15" is small, almost violin sized. 15 1/2" is manageable for most people. 16" is fairly big, and 16 1/2" is too big for many men!