How do I take a drawing and colour it on the computer?

So I created a drawing (its just cartoon, nothing extravagant or super detail just a little bit of criss-cross shading not blending) and i want to get it printed on to a 50'' x 60'' fleece blanket at Walmart. The only problem is, is that its black and white, and if i colour it, it wont turn out well on a fleece blaket. I want to know if there is some program (other than microsoft paint) thats free or somewhere i can get my picture recreated on the computer ( make it almost identicle or better) and have it opaque with bright colours so i can get it printed and scanned succesfully at walmart so its picture is perfectly printed on the blanket. Thanks for the help!!