After many years in this biz I can without hesitation tell you that in order to get proper pest control you must do these things in this order.
1. Identify what pest you have with the thousands of resources available (Books, County extension agent and of course the internet.)The arbitrary application of either the wrong pesticide or the improper rate can often lead to an explosion of pest population. Use of a simple magnifying glass or microscope is essential in ID of a given pest.
2. Select the proper pesticide for its intended use. Self explanatory. Applying say a fungicide (disease control) for a something as simple as aphids will offer no level of control and will likely be a waste of money and time.
3. Read the label ! Read it as many times as it takes until you fully understand every portion. The MSDS sheet (usually also available online) can be somewhat complicated, and ,often includes info.that does not pertain to your particular situation but would only be helpful to a particular professional in case of an accidental overdose or reaction.
4. Apply the product only as the label directs. Applying the product at less/more than the label instructs can lead to less than desirable results. The most common mistake among non-professionals is over-application. The old motto of "A little is good must mean a lot is better" has been repeatedly shot down in the case of using pesticides. Many excellent pesticides that took millions of dollars to research and develop have been removed from the shelves not because they were'nt effective or, because they were harmful to the environment. Simple misuse by people that didn't take the time or, lacked the intuitive to read and follow the label lead to many problems. I could site dozens of examples.
5. Follow up. Check your plant(s) frequently. Pesticides can only work for the time they were intended to. Rotation with another pesticide that has a different active ingredient but, is also labeled for the given pest you are trying to control is a good method to help prevent species resistance. Constant bombardment with only one control method can eventually lead to a population that cannot be controlled no matter the product used. Finally, learn to accept some loss from pests as part of the consequence of growing. I have yet to read of a chemical that offers "total eradication" for any problem. Acceptable levels of control should be what you are shooting for.