What is the pest that you have?
You can't choose the right treatment without knowing what the problem is. You wouldn't want your doctor to treat you without knowing what problem you had. You should think the same way about treating your garden, lawn, or home with a pesticide.
Is the pest really a problem?
Is the pest causing actual damage to the plant? Will the treatment be more of a danger than the pest itself?
Is there a non-chemical solution?
Are the natural enemies of the pest already present in the environment? Could you hand pick the pests off the plant? Could you pull the weeds out by hand rather than using an herbicide?
Is there a least-toxic chemical that you could use?
For example, if you are having problems with caterpillars, could you use Bt (Bacillus thuriengensis)? It's very effective against caterpillars and yet is very, very safe for humans, other mammals, birds, and fish. Could you use a horticultural oil? Or insecticidal soap? These are very safe for non-target organisms.
If you need to buy a pesticide, how much do you actually need?
If you buy a concentrated insecticide but only use a small amount of it, wouldn't you have saved money (and potential environmental dangers) by purchasing a smaller amount? You should not purchase more pesticide than you will use in three years.
Once you're out there with your pesticide, remember to read the label and follow all directions!
In the United States, the label on a pesticide is the law. If you don't do exactly what the label says, then you're breaking the law. Do not think that if a small amount is good, more is better - that is not true! You're breaking the law and you could be putting yourself, other people, animals (including pets), and the environment at risk.