Finding the best way to practise is largely an individual thing and may differ according to the specific piece and technique required. Your teacher will show you things to look out for, and suggest the best approach to use. Here are some general guidelines that can be applied at any level:
Before learning a new piece, spend some time studying the score. We want to begin to discover the message that this music will convey. Firstly there will be clues in the title, in speed and style directions, and in the notes themselves. After logging the preliminary things, like the key signature and time signature, look for themes (repeated phrases and patterns) and the overall format of the piece. This will help in breaking it down into manageable sections to work on.