When you fertilize, you are attempting to add or replace nutrients that are missing from your soil. So, the first step in deciding how to fertilize is to try to determine what is missing. If nothing is missing or in short supply, then fertilizing will do no good, at best, and can actually be detrimental.

There are multiple ways to determine what's needed. One primary tool is a soil fertility test. Your local ag extension office should be able to help you with that for free or for minimal cost. Another way to determine what's missing would be to watch for signs of malnutrition (slow growth, certain leaf discolorations, etc). Now, you will have to know what to look for with that last method. A combination of methods may be the very best way to accomplish this task.

Too much of a good thing, can be a very bad thing. Think of fertilizer as vitamin pills. Let's say you want to recommend that a friend of yours take more vitamins. You know your friend is getting some nutrients already (he/she is living) and it's possible that they are eating well-balanced meals or even already taking a multivitamin (your plant's soil already has nutrients in it). Obviously, just giving them a multivitamin (on top of whatever they are already doing) is not necessarily a good idea. Afterall, some nutrients can kill you if you take too much! The same is true for plants.

Another thing to look at is that plants generally have an "optimal window" of growth. If they grow too slowly, they are sometimes unable to respond to or outgrow potential problems and diseases. BUT, growing too quickly is also not good! Plants with excessive growth can become lanky and are frequently magnets for pests and diseases. Keep the "Goldilocks Principle" in mind when working with your plants!

While we're on the subject of fertility, I must mention compost. Compost is often described as gardener's gold. It's a great way to add nutrients and improve the soil's tilth. It would be hard to add too much of any nutrient with compost, but a little compost can actually do a lot of good. I think of compost as a "safe" fertilizer. Obviously, you could over-do the compost thing, but you'd almost have to be trying to. Adding compost can be as simple as using shredded wood mulch and letting it break down over time, or as stereotypical as having a compost bin and adding cuttings, tablescraps, etc to it. For most cases, I'd recommend adding compost to the top of your soil and letting nature take care of incorporating it into the soil beneath. If you have fresh mulch and wish to add compost, you could always pull back the mulch and add the compost before recovering the area.

As for what is "ailing" your plants, it's anyone's guess without information! Pictures and as much detail as possible might give us some clues.

P.S.

I forgot to say, this is probably NOT the time to fertilize most plants, even if you determine they need some nutrient(s).

Also, fertilizing sick/stressed plants frequently exacerbates the problem.