From a practical perspective, teaching yourself guitar may be the only option. Guitar lessons cost money. If you’ve just scrounged your pennies together in order to afford a decent starter guitar, you may not be eager to throw down another twenty bucks a week or more. Compound this with the fact that most guitar instructors have their own methods and will teach you at their pace, not yours. This makes sense from their standpoint, but it could get a little frustrating and expensive from your point of view.
The simplest answer and the one that no student that ever wants to hear is practice. Changing chords is the process where many beginners fail, and quit. But after that, the rewards will be simply impressive. There are a few tricks to get a chord transition to happen faster. Use a metronome: Set it on four beats and set it as fast or as slow as you want. Then get a chord in your mind, say D. When the metronome reaches its last beat, press down the strings. When it happens again, strum it and let it free. Then again. Do this 10 to 20 minutes a day and in less than a week, the chord progression will begin to sound much better.
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.