Tomatoes are some of the most popular and widely grown garden plants. This is probably because tomatoes are quite easy to grow in most cases. Usually you can buy them in small 'flats' or grow them from seed. There are many different varieties, and which one is right for you depends on where you live and what you want out of your plants. Tomatoes generally don't require very much water; too much will decrease the flavor quality of the fruit. It is best to water deeply and infrequently. They generally need to be trellissed or trained onto something, or else the plants sprawl on the ground and the fruit rots.
Tomatoes generally get infested with insects and fungus late in their life. The most interesting (and annoying) of these pests is the tomato hornworm, which grows into a quite large caterpillar with a horn on its butt, which is a voracious feeder. You can hand-remove them at the start, but usually their arrival just signals the old age of the plant which probably won't be productive for much longer anyhow. If you've planted several plants you will be sick of tomatoes by now anyway.
Many tomatoes are grown in California; most ketchup comes from California's Central Valley. In the late summer, rural street corners and onramps are splattered with tomatoes which fell out of the trucks as they turned. I've even been hit by flying tomatoes while driving on highway 5 before.