#1
Hey.

When I am practicing sweeps certain fret like the 12th fret keep ringing a note after being played. I don't think i am muting the strings properly. I've looked online for the best technique but haven't really found anything very helpfull.

Thanks in advance.
I will shred in the end!!
#2
If it's only certain frets then it's your guitar not you... Give it a proper set up and you should be okay.
Gear List:
B.C. Rich NT Jr. V (With Seymour Duncan AHB-1 Blackout in bridge)
Electro-Harmonix Metal Muff
Marshall MG15DFX
Jazz III picks
DR strings
Planet Waves Cables
#3
It's not your guitar. Notes on 7th, 9th, 12th, 15th,... etc fret ring out, because those are the places where you find natural harmonics.
You are correct; you are not muting them correctly. Especially an arpeggio like:

12
12
12
14
14

is quite difficult to play cleanly on high gain. Just practice slowly, and with time you'll get it down.