Jenunruh, I am always happy to hear that someone is getting into gardening, and asking good questions about how to succeed. There are many different ways to garden, and different reasons too.
I've never used anything stronger than insecticidal soap on outdoor insects, and not very much of that. So, I'm not the one to recommend an insecticide nor a fungicide.
There are, however, some general pest control principles I'd like to share with you.
First, look for plants that are disease and pest resistant.
Learn to tolerate some damage.
Always identify the pest before treating. You don't want to wipe out the pollinators, butterflies, pest predators, and the beneficial mycorrhizal community (among others).
Cultural and mechanical practices can go a long ways.
If you do decide to use a chemical treatment, look for the least-toxic choice.
Again, make sure you have identified the pest you are dealing with, then choose the least-toxic alternative.
Always read and follow the label directions.
Here's a link to some information from University of Florida Cooperative Extension:
(Get acquainted--they're a great resource.)
Ask Big Questions!
Here, marigolds are generally grown as annuals. Plant them in the Spring, enjoy through Summer, they will die in the Fall. Relax, treat them as annuals, and find what else does well for you.