Move the crossfader on your mixer so that only one deck can be heard through your speakers.
Set the pitch bend controller on this deck to "0" and play a track
Set the headphone controller on your mixer to monitor your second deck and place the headphones on your head. Make sure you can still hear the track playing through your speakers. Only put one headphone to one of your ears if this helps.
Find the first beat on your second track. You can do this manually if you're playing vinyl, or you may use the flywheel or virtual flywheel if you're using CD decks or a digital mixer, respectively.
Cue your second track in at the start of a 4 or 8 bar phrase of your first track and use the pitch bend controller and flywheel to adjust the speed of the track plying on your second deck so that it matches the first. Use the bass drum as your main guide point. As you improve, you should be able to sync tracks even in the absence of a beat.
Slide the crossfader to a central position when you're confident that the beats of your tracks are in perfect sync. Both tracks will then be channeled to your speakers. You can re-cue the second track before doing so if you want a different start point. Allow the tracks to play in tandem and choose a good point to fade the first track out.
Stop your first track and prepare another to mix into your second.
Keep practicing. Once you've done it for a while, mixing will become second nature.