Practice at least 20 minutes 6 days a week. Consistent practice will allow you to improve your guitar playing skills over a short period. Taking long breaks and not staying dedicated will wear away your muscle memory and delay the development of your skills. Instead, dedicate 20 minutes to an hour of practice throughout most of the week. During practice, you can either run different drills or you can try covering a song.
You do not need long fingernails to play guitar. In fact, it's best to keep your fingernails short! As for the callouses, these may be unavoidable. If you would like to avoid them as much as possible, ask your guitar instructor about lowering the action of your guitar (the action is the distance between your strings and your frets) so that the frets are easier to press down on.
Try listening to different styles of music for inspiration as you learn to play. If you're trying to learn to play by ear, listen to the blues and try to mimic that sound. Blues progressions are especially great for learning to play guitar because it's built on the basics of music theory. Once you learn those progressions, you can start practicing them in different keys, as well.
Beginning Guitar Class - Open to all beginners age 13 and up, or intermediate players who want the review. Begins with holding and tuning your guitar, names of guitar parts, and quickly progresses to chords, scales, tabulature, and playing basic songs. A variety of styles are introduced to encourage students to explore and find the styles they like best. Classes are organized according to age and skill level, so it's an easy way to make new friends too. Classes start every month and may be repeated as often as you like. Cost is $70 per month (4 hours).