At the beginning of the game you need to be careful about cross-pollinating. Until you are able to purchase better soil, most hybrids will lose health steadily until they die. The safest thing to do is to pollinate only very similar or identical plants and sell them until you have enough money to improve your soil. You can usually find at least one common plant that will sell for upwards of $40, so it won't take too long to get better soil. As your soil improves, the number of more valuable hybrids that you can grow will increase, and you'll be able to make money a lot faster.

Also, it may seem obvious, but you need to make sure that you aren't letting your plants run out of water or they will die.

If you're still having trouble, let us know and we'll try to help figure out what's going wrong.