Tarantulas eat anything smaller than themselves. Insects and occasionally a reptile, small mammal or amphibian will be on its menu often. Arboreal tarantulas sometimes even catch a bird out of its nest. Most species will wait in their shelter until a prey comes by. Some prefer to hunt for prey. In nature, it happens that a tarantula will live a few weeks without food. Feeding your tarantula every week or every two weeks will be enough. If you want it to grow faster, you can feed it more often. Just before a molt you will notice that the tarantula stops eating, this is normal. Tarantulas can survive several weeks or even months without food, as long as they get enough moisture. Note: Grammostola species are known for their food refusing, some will stop eating for over 6 months before accepting food again.
 
Insects you can feed your tarantula with:
1. Crickets: available from 2 to 30 milimeters
2. Grasshoppers: for sale as young and adult, they are bigger and stronger than crickets, but I noticed that some of my tarantulas don't like to eat them
3. Fruit flies: perfect for your small spiderlings but they can escape quite quickly
4. Buffalo worms, flour worms and morio worms
5. Cockroaches: very healthy and full of protein

Mammals or meat you can feed your tarantula with:
1. Baby mouse: one day old mice, it can be fed either frozen or alive
2. Adult mouse: I prefer it to be dead (frozen), because it can hurt your tarantula
3. Beef heart or chicken stomach: best way to feed your tarantula when it is very weak, feeding by a pair of tweezers
Note: Don't feed mice too often because they contain a lot of calcium. Calcium may make it harder to molt.

On the other side, it's a very nice boost-up for your tarantula if you feed them mice shortly after they molted.

 

Things you can NOT feed your tarantula with:

1. Beetles, except buffalo-, flour-, and morio beetles

2. Catterpillars

3. Stinging insects, such as wasps and bees

4. Flies

5. Other spiders

Just never, I repeat, never give your tarantula wild caught food. This can contain parasites, diseases and pesticides.
 
A tarantula also needs moisture to survive. Part of the fluid is achieved from the prey, the other part it needs to take it by itself. I use glass candle holders  IKEA of 3 cm diameter in my big enclosures and I use small plastic cups in my smaller enclosures. Also you have to spray the enclosure once or twice a week, but note that you don't spray directly on the tarantula itself.