Just to add on all the great advice here. When you feel you've properly fretted a chord (maybe a D as an example) individually pluck each string seperately. Each should ring on its own without any interference.

If there is, you might be accidently mutting strings with parts of your finger. If its not that then its probably just finger strength which only improves with time and practice.
Alternatively (unlikely) the action on the guitar is too high. Thats the space between the string and finger board. Too high action makes it very difficult to fret the string and get a clear sound.

I'd safely say its just a matter of finger strength. Hang in there, it gets easier ;)