One of the most useful tools to use when you are practising is the Metronome. If you weren't sure, they are the old pendulous looking thing that when tick- tick-tick on your music teachers desk. Nowadays they are small electronic boxes, that are very accurate and some have fancy features like accented beats and subdivisions of the beat. I personally recommend the Korg MA-30 metronome, it has many features, is not expensive and a good build quality.
How and why to use a metronome
Metronomes help you play in time. This is probably the most important aspect of playing guitar so it is one that you should try to get right at the start. A good way to start getting used to working with one is to set the beat to about 50 Bpm and strumming some chords evenly with a downstrum on every click. Then try playing the up in-between the beats as well, so that you strum down on the click and up evenly inbetween. See the beginners lessons on strumming.
Lessons that use the metronome will explain how to use it for that particular technique.
Try to set it and play some songs along to it. Try to feel the groove, feel at one with the beat. Sounds silly, but it is really that. Try to feel the beat and be with it, inside it. If you have a drum machine or play along CD's then they are just as helpful, but you can't speed up CDs like you can a metronome.
The metronome that I use (and most of my students) is the Korg MA-30. It has many useful features including, tap tempo, headphone out, volume control, beat subdivisions, LCD display and the tempo can be changed up or down 1bpm. The link below will take you to the metronome at Musicroom.com - a reliable and good business based in the UK