The guitar is a remarkably hackable instrument for a million reasons that will be revealed to you as you spend more time with it. As you go along in your journey you’ll find a million shortcuts and fun ways to learn fast. I’ve rarely heard any of this stuff from guitar teaches, so beware, trust your instincts, and learn from people who can show you where you want to be.
This is not a substitute for learning scales. At some point down the road you are going to want to dive into music theory at least a little bit, if nothing else so you can understand how scales work and how chords are built. For now you are a beginner, and this exercise can go a long way toward loosening up your fretting hand and improving your technique.
When you play the right note or chord, you will hear it. It will be a pleasant sound, it will be clear and won't have any blocked string sounds (unless it needs to, but you'll get to that later). To play a note or chord correctly, you have to get your fingers in the right position and press as hard as you can on the frets; it will become a habit if you practice enough.
As a beginner, it is best to avoid making a pawn shop, flea market, or yard sale purchase unless you are shopping with somebody who has experience in purchasing a guitar. Having never purchased a guitar, you may not know what to look for in terms of damage or guitar quality. With that being said, a local music retail shop is your best bet. You may also shop online with a reputable company, although be sure to review the business’ return policy.