The thing is, everyone says to practice with a metronome... But when doing so, playing scales for example, are you supposed to play one note for one beat of the metronome and gradually increase?

Or just find the pace, and go however you like still keeping the beat. I'd be very glad for help, I just started playing guitar and I've never played any instrument before, so it'd really help me out if you could assist.
its meant for you toget used to playing in time. So when playing with your band you dont keeping jumping tempos.
get used to this (only if your playing rock because these rules only aply in 4/4 time) play the scale as one note every 4 clicks. (ie one-two-three-four) then play each note as 2 clicks (one-two three-four) then play one note per click (one two three four). Now you have covered the longer notes (whole, then half, then 1/4). Move on to two notes per one click (1 and 2 and 3 and 4 and). and finally do 4 notes per one click (one e an a two e an a 3 e an a 4 e an a). In case this is confusing the "one," "two," "three," and "four" always fall on the "click" from the metranome, the "and," and the "e an a" fall evenly spaced between clicks.
Ah, I understand now. Thanks for clearing that out! And yes, I play classic rock, so it should help.
I've never used any metronomes than other the one I got which is Boss TU-80 Tuner & Metronome. Not sure if it's different to any others around. Anywa, you set the rhythm of the ticking and the tempo.

Once your comfortable with that try playing in triplets with three notes every tick of the metronome, start off slow and build-up, this is great for memorizing scales and jumping between them.