Summarize:

To print on a t-shirt using a stencil, you only need a few basic supplies. You may have most of them around your house. If not, you'll find what you need at a craft or art store. Round up the following materials:  A t-shirt. A plain, basic cotton t-shirt is fine. Know that some paints and inks will bleed through thin cotton, so if this is a concern for you, you might want to choose a thicker blend. The color you choose should be light enough (or dark enough) that the paint colors will show up well. A stencil. You can buy a pre-made stencil from a craft store or  make your own using cardboard. Paint or ink. Acrylic fabric paint is a good choice for printing on t-shirts. You could also go with a fabric ink or stain. Look for a type that won't come out in the washing machine. A small paint roller and paint tray. You'll need this to apply the paint to the t-shirt evenly. If you don't have a roller, you can use a wide paintbrush. Tape. This is to hold the stencil in place while you apply the paint. Masking tape works well for this purpose. Cotton t-shirts shrink in the wash, so it's important to run them through the washer and dryer before you print on them. If you wait until after you print, the design might end up getting warped. When the t-shirt is dry, iron it to remove any wrinkles. Lay out some butcher paper or scrap paper on a hard, flat surface. Place the t-shirt on the surface and straighten it out so that it has no folds or wrinkles. Place the stencil on the part of the t-shirt where you want to print the design. Tape the edges of the stencil to the t-shirt to hold it in place.  If you're worried the paint is going to bleed, place a piece of cardboard inside the t-shirt; this will prevent the paint from seeping through the shirt to the other side. To prevent paint splatters from getting on your nice clothes, you might want to put on an old t-shirt before you start painting. Pour the paint into the tray. Roll the roller over the paint a few times so that it is distributed evenly around the roller. Do a little test roll on a piece of paper. Using firm, steady strokes, use the roller to fill in the design in the stencil. Cover the entire design and overlap the stencil by an inch or two. Take care not to accidentally apply paint to areas outside the stencil. Carefully lift the stencil from the t-shirt and set it aside. Now wait for the paint to dry completely before touching the t-shirt again. Once the paint is totally dry, place a clean cloth (such as a thin dishcloth) over the design. Set the iron to high and iron over the painted area of the t-shirt. This helps set the paint in place so that it won't easily come off. You're now free to wear your new t-shirt. Wash it by itself using cold water the first few times it gets dirty. With time you should be able to throw it in with the rest of your laundry.
Gather supplies. Launder the t-shirt. Set up your work space for printing. Prepare the roller. Paint the t-shirt. Lift away the stencil. Iron the t-shirt. Wear and wash the t-shirt.