How to practice can be a very elusive concept. I think one has to know why practice is so important to playing an instrument to actually practice well. A large part of playing an instrument requires developing good reflexes and muscle memory. The process of learning a piece of music is to first know what to play, which is a mental thing, and not that hard to do since the brain is used to doing this type of memory work. If playing a song just required this, there would be no need to practice, you would just memorize the notes, chords, etc. and play it. The real work is training the muscles to play the notes smoothly, quickly, and automatically. Unfortunately the muscles don’t learn as easily as the brain, so they need to be shown things over and over, this is what practicing really is. Music and sports are similar in this way, both require practice, since so much of it is based on muscle memory, the more you do it the better you get.
When you first pick up your guitar it’s a good idea to ease into playing by starting out with scales, arpeggios, etudes, or a simple song. At this point you want to get the hands and muscles moving smoothly and accurately. Focus on getting a smooth rhythm, good tone, and really connect with the notes. Start slow, it should be easy, if it’s not your playing to fast, When you feel in control, then start building speed, this is a great place to use your metronome. Once things are feeling good you may want to just play some tunes that you know, like, and feel comfortable with, in other words just play for the fun of it.
A good idea here is to establish a goal that you think you can achieve in the week between lessons. This would be based on how much time you have to put into practicing. As far as learning a difficult tune, you may want to run through it once or twice, but you’ll do better if you focus small sections, go for quality not quantity. You can set up a routine were you repeat a section a set number of times. Most people try to accomplish to much during the week and end up accomplishing very little, which is discouraging. Set attainable goals and you’ll see steady progress, which is a lot more satisfying.