The viola is 5 notes lower than the violin and has a richer tone.
At full size the violin is smaller and has a brighter tone. However, if you are small and want to play the viola, small violas are available or I can convert a violin to a viola very easily in about 10 minutes.
I can advise on a suitable size and can recommend retailers of instruments. I may be able to lend you one while you find one to purchase.
You can learn both violin and viola, but I recommend becoming proficient on one before starting on the other.
Please look at my YouTube video 'Violin or viola?' to help you decide.
There is a lot of great music for both, so go for the sound you love best.
You can enter for Trinity and ABRSM exams or Music Medals if you enjoy having a goal, and I can play the piano to accompany you in your exam or medal test.