I agree with Maride's advice and will add mine. Use a stabilizer on the inside of the shirt to keep your shirt from distorting. A heavy wash-away will work well because it will allow the t-shirt to drape naturally when you finish.
If you paint the fabric, use a textile medium in any acrylic paint and have at it. When painting fabric, less is more. Heat set after it is dry and you are golden. There are fabric paints designed to be used without the textile medium but I liked using what I had on hand. I've painted a LOT of shirts.