For digital art do I need a graphics tablet or am I ok with a mouse?

Its worth getting a tablet I'm rocking a Ugee EX07 its great for the price like 70 bucks on amazon and its a 6 x 8 area.

You will have an easier time adapting traditional paper techniques with a tablet. A mouse is not pressure sensitive. Also, Monoprice has great tablets, at 1/3 the cost of Wacom. I use one. It's great!