Do I need a metronome? A metronome can be a great tool to help with your practice, it can help you to make sure you are playing ‘in time’ which is a very important part of playing music. But…be aware that if you are just starting to learn an instrument like guitar there is a lot of things to think about, (are my fingers in the right place, are all the strings sounding clearly, have I started strumming the chord from the right string etc. etc.) Adding the extra pressure of a metronome at this stage might actually be more confusing than anything else. So if you are a total novice I would suggest steering clear of metronomes for now. If on the other hand you are able to play all your basic chords and are able to move between them without too much difficulty and need to test yourself on the chord changes then a metronome will be of great benefit.
If you are at a stage in your playing where you are studying scales then a metronome is a must and will help you gauge your progress. Start off playing your scales in 1/4 notes (crochets) at 60bpm then try the same scale in 1/8th notes (quavers) at the same bpm then try 1/16th (semiquavers) again at 60bpm. If you can manage this without any errors (not just wrong notes, also fingering, tone, right-hand coordination) then increase the tempo to 70bpm and try the same process increasing the tempo until you find your limit – the point where errors start to happen. Now you should have an idea what you need to work on. Go back to a slower tempo and work on the element that needs attention (wrong notes, tone etc.) and next session try again until you can play at a higher tempo without errors.