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 ...

I am often asked "what are the basic things a beginner should learn".

Well, whether you are learning guitar or ukulele there are some things you really need to learn. Here is my basic learning list:

1. know the note names of all the open strings
2. know the numbers assigned to each string from top to bottom
3. know the fret numbers
4. know at least some of the instrument part names. e.g. saddle, nut etc
5. know how to tune the instrument with and without a tuner
6. know how to change strings
7. know then names of all the notes covering at least the first five frets
8. know the naming convention for the fingers of the right and left hands
9. know and choose how to hold the instrument
10. know how to read chord diagrams
11. know how to read TABS and the basics of traditional music notation
12. know how and where to place your fingers on the fingerboar
13. know how chords are built including major, minor and 7th chords
14. know some basic scales
15. know something about chord progressions
16. know ...