When mixing sound, check out the size and acoustics of the room. The acoustics for live sound in a stadium will be far different than those of a coffee-house. You don’t want to overwhelm your audience or make mixing for studio sessions difficult. In a smaller room, amplifiers and drums are easy to hear naturally. If you pipe them through a PA, it will make them sound messy and unclear. You won’t get the right flow and the drums and amps will drown out other elements.
You’ll need to make sure the vocals are loud and clear at all times. Vocals will not be naturally heard over drums and guitars so try a technique such as subgrouping. This allows you to compress the vocals all at once and put them in their own group, thus making all microphones easier to adjust and control. Remember to account for monitor feedback as well; you'll need to run several practice sessions before you can eliminate this problem and get the best sound.
Drums tend to be extremely difficult to mix live, so first check to see how they sound naturally in the space. In most smaller areas you’ll only need to amplify the kick drum, so try to mic the kick drum and the snare. If you need to mic the toms, use one mic for every two toms. This will free up mics and space while still giving a full enough sound. Unless you’re working an outdoor venue, chances are the overheads and cymbals won’t need any amplification, either.
You probably won’t need to mic the amps for bass and guitars. In many cases, you may actually need to lower the volume on them due to their intense sound. If you’re using an acoustic guitar, you’ll either need to mic the guitar itself or use an acoustic amp if it's available.