Thanks to you guys, BrandonBass yes I know how to keep time right, thanks the metronome for that

and mKettner I know it's very good to be able to play everything on an instrument and eventually I will learn more technique but for the moment I'é working with a pick becasuse I play in a grunge band and it's the sound we want so it is my #1 concern for the moment. About how to mute the strings I've got to say that this is something I'm not sure about. When I play something simple this is not a problem, but when you something with melody like picking string 4-3-2 on fret 1-3-3 (like a power chord on guitar), do you want to mute each string before playing the other or do you let each string resonate. A good example of what I'm talking about would be the song Sappy from Nirvana. Thanks for the help.
Salut