I really found C to be difficult for weeks. As a fellow newbie, I have some advice for what helped me get this one., but definitely listen to Bill MG and watch his video clip and try whatever he suggests any time he answers someone!
So,a combination of several things helped me get it.
1) I adjusted my hand and finger angle. I was trying to fret the chord with my fingers parallel to the frets - you know, 1 finger 1 fret rule and all that. It's a really hard stretch that way and it puts your arm, wrist, and fingers in an awkward place. If you're doing it that way, try to angle your hand a little so that your fingers come across the frets at somewhere between 30 and 45 degrees with your fingertips angled towards the bridge. When I hold the guitar with a C chord fretted, my fingers point back towards my face, but probably about at my left ear instead of full-on face. I guess my fingers actually point almost sttraight up my left arm. If you look at your hand you'll notice that if you have a slight angle then your fingers are able to reach further up the neck because you are able to use the length of the fingers instead of just trying to spread them apart. At first I thought I wasn't supposed to do that, but watch people play and you'll notice that hand and finger angles change constantly.
2) I was not arching my fingers enough, so what they call bad finger posture. Everyone says "play on your fingertips" and I definitely got that and thought I was doing it, but doggone it if my fingers were still too flat. Honestly, I have not heard anyone say "ARCH your fingers" and for me that was the phrase that sunk into my thick skull. I think the angle of your hand and fingers comes into play with this too, actually.
3) Bill's suggestion on using the thumb to mute the low E string is good, and I think letting that thumb come over the neck helps with the hand posture too. For me, my thumb comes out dead even with the first fret wire.
4) An alternate way to mute that low E string is to let your ring finger that is on the 3rd fret of the 5th string (playing the low C note of the chord) slide up just enough so that you are touching the low E string with that finger. I have noticed that if you do this, it actually allows you to slide the middle finger that is fretting the 4th string up a little bit too. This gives a tiny scintilla more clearance between the 3rd and 4th string, so you are less likely to mute that third string with your middle finger. This nudging the 4th and 5th string up a little bit actually allows your fingers to be a little more flat without causing a problem.
I have to throw out the disclaimer that I've only been doing this for a few months myself, so if anything I'm saying here sounds like it might not be good advice I do hope someone with more experience will correct me. These are the things that did help me get the C chord working. I ddin't do all of those things at once. It was just a little adjustment at a time, then another tiny adjustment, and eventually I looked down and noticed that my hand was in an entirely different position than where it was when I started!
Hope this helps! Enjoy the journey!
It takes as long as it takes unless you quit - then it takes forever and you will never get there.
