How to practice? and How long? Well that will depend on your level, goal and motivation. However, it is important to at least be consistent.
All it takes is to get up, get your instrument (if you can pick it up, do not try with a grand piano ;) ) and sit down to play. A five minute practice session can easily turn into hours or fun and enjoyment.
Long periods of practice does not necessarily mean a better outcome. Be conscious of your practice, warm up and take care of our body. Practicing like a robot will not result in a genuine performance.
Your music is YOU and how you play will reflect on the effort you put in, not only in practice but also the attention to detail and emotional investment you transmit through your hands. Your instrument does the talking but you must make it speak!
A good teacher should guide you and give you a practice plan to help you improve. Ask them to give you a practice schedule and to be sure to include technical (not just scales and arpeggios). The more patterns you do, the more variety you play, the better you will become!
Here is a FREE DOWNLOAD poster to keep near your instrument at practice time:
The file is free to share via creative commons license.