Tuning is about as fundamental as it gets, in fact it's so important that I would go so far as to say – if you're not in tune – don't play!
The more you play, the longer you play and the closer you attend to sound, the more you will be able to hear finer increments of pitch and because of this, as a beginner, it's common to think you're in tune when you really aren't. If you always play in tune, you'll have a frame of reference so that you can begin to tell when you're out of tune.
I'm going to give you a few tuning exercises, I recommend you do at least one per day. After doing the exercises, check how close you've gotten with a tuner (a high quality, clip on tuner)
Exercise #1: tuning off of open strings...four of the six open strings can be matched to the adjacent string below at fret number five (the exception is the B string, the matching note on the adjacent string below is at fret number 4) The other exception is the low E, since there is no lower string you can match that string to either the seventh fret of the A string or the second fret of the D string (those E notes are one octave higher than the low E)
Exercise #2: comparing fifths fret to open, adjacent strings above... Again, this works for four out of six strings. The B on the G string is the exception, this is at fret number four. Finally the A on the high E string can be matched to the tenth fret of the B string.
Exercise #3: tuning open strings to a note two strings higher...The low E will match the D string second fret, the A string will match the G string second fret, the D will match the B string third fret, the G string will match the E string third fret and the B string will match the seventh fret of the high E string.