Have you typed something into a search engine only to be presented with the “did you mean?” or “searching for … instead”? Your spell check can do the same thing to you, and it can have other issues as well. It’s a good idea to use it, but don’t always trust it.
There have been more than a few instances when I knew for certain that I’d spelled a word right, only to get the dreaded red squiggly line under it. When I looked at the suggestions, they were for other words I didn’t want. If I trusted spell check without question, I’d end up ruining a lot of my writing.
Spell check isn’t perfect. Just accept that. It can help, but your best bet is to practice on words you have trouble with so that you can spell them better. If you’re not sure that spell check is steering you the right way, you can also look the word up in a reputable online dictionary.
I’ll also warn you against adding words to your spell check dictionary. It’s far too easy to add a word you think is spelled right but isn’t, and then suffer for the error later. Unless you are 100% sure beyond any shadow of a doubt, don’t add words to the spell check dictionary. Not only could you add misspelled words, it’s a pain in the backside to get words out of those programs.
Reading well-written work and practicing words you have trouble with are usually the best ways to learn to spell better. If you have trouble with a word, use it frequently so that the proper spelling will stick in your mind. Be sure to read both factual and fictional work too, since both use different words. You can learn a lot of vocabulary and spelling from well written books in almost any genre.