Fabric painting is a lot of fun, and quite often you're smitten by the bug when you don't have the right paints to hand.
Fabric paints are quite expensive to buy. If you want a small amount for a craft project, or you want to let your children loose on some t-shirts, then you don't necessarily want to spend too much.
Firms like Berol sell a Fabric Medium, which is an opaque liquid that can be mixed with ordinary paint to make it suitable for painting onto fabric. Use small pots like yoghurt pots, or paint mixing trays to mix up small amounts of paint at a time.
Acrylic paints can be used directly onto fabrics, and should be successful. This is because they have a plastic element to them. You can make it thicker with the normal acrylic thickeners. Acrylic paints come in a huge range of colors, and can be used for other projects afterwards.
If you add PVA glue to your paint it will make it very similar to acrylic paint, and it can be used on fabrics. You can add glitter to make it sparkle. Don't mix up too much of this at once, as it will start to solidify round the edges. If you do want to mix up a lot, cover it with saran wrap (cling film) until you are ready to use it.
None of these can be ironed. You will also need to be very careful about washing out your brushes. They can be washed with water, but you should do it before the paint dries.
If you have some commercial dyes around the house, you can make these up into a solution and use them to paint onto the fabric. If you don't, then you can use home-made dyes. There are several Helium articles on this. Try onion skins for orange or beetroot for purple. Just think of anything that will stain fabric! These dyes aren't usually very dark. Check out the ones that need a mordant to fix them - without this additive they will come out in the wash. Be careful not to splash it on any of your best clothing.
I think that commercial fabric paints are worth the money in the end, but if you need a quick solution, then any of the above things will do.