There are some rules almost every guitar player must adhere to if she, or he, ever wants to become proficient at playing the instrument.

1. Practice. Yes, of ourse, it is something everybody either says they do - or will, do - but I'm telling you, it is vital. Play every day, even if only for 10 minutes - and if that is impossible, due to holidays or similar, then make up for it my playing longer for a few weeks. Don't play until your fingers are too sore, but make the effort.
2. If you use a plectrum, practice playing down-up (and up-down) on every strings as well as straightforward all downs!
3. You need to build up the muscle power in your left hand (right is you are left-handed), so play runs of 4 frets, one after the other, both ascending and descending. Do it on at least the top four strings. It is hard to do, especially descending, but keep at it - and make it a part of your daily routine.
4. Practice the "pinch" on your plectrum so it never falls out of your fingers. 
5. Fingerstyle players will find it helpful to practice thumb, 1st and 2nd fingers picking Plus any combination of thumb and fingers 1, 2 and 3 - including all of the for extended arpeggios.
6. Last, but not least, practice chord changing, up and down the fretboard. Newbies will probably hate barre chords - especially the "F" shape fingering - but do force yourself to keep attempting to do it - and then be amazed at how easy it turns out to be. Obviously youngsters and people with small hands have limitations, - but still, do your very best, do at least try!

I can't stress enough the need to discipline yourself to practice. It is normal to "hate" some of the routines, but in the end you'll be pleased you made the effort - as will those who listen to your efforts!