Beginners, if you find changing chords quickly and cleanly hard going, read on.

The first thing you should do is try to memorise the chord shapes you wish to play in a piece or exercise. Next you need to hold down each of those chord shapes as cleanly as possible so all the notes play and sound as they should. At first you may have to deliberately place each finger on the correct string and fret and repeat the process when the chord changes - which is quite normal - but in time, and once you can easily recall chord shapes from memory the basics of this process will become more natural to you and your concentration can then be more focussed on how well you play the chords.

So, starting with a couple of chords which you feel comfortable playing and have committed to memory, play one a few times and then repeat the process with the second chord, and then back again and again and... Whether you are strumming or fingerpicking with your right hand (left for left-handed players) try to keep the same pattern going rather than stop while you move your other hand to change chord. Initially keep you fingerpicking or strumming simple (down strokes are fine). At first you may think you will never get the hang of changing chords, but believe me, you will. 

Once you have "mastered" the two-chord changes try adding a third, and then move on to do the whole process again using a new set of chords. The common chords to use will likely be: G, C and D(7); D, G and A(7); A, D and E(7).

One other thing, if you find the two and three-chord exercise really hard going take a break from playing - in fact it's a good idea to take a break from any exercise which you have problems with and return to it later, even the next day. But if you aren't quite ready to stop the session entirely you might just try the "extra" exercise of changing between two chords which share most of the same string and fret positions, for example C and Aminor. Typically you will play the C-chord as X32010 (i.e. standard TAB format referring to strings 6,5,4,3,2,1 in that order and where the "X" means the string isn't played) and the Aminor-chord as X02210 which means you only move one finger to make the change (i.e. 3rd finger moves from the 3rd fret of the 5th string to the 2nd fret of the 3rd string) and you play the same five strings in each case. Another easy change is from G to C in a style which is commonly used by singer/players. In this case you would play a G-chord as 3X0003 with your 3rd finger on the 6th string and your 4th finger on the 1st string, deliberately "muffing" or damping (X) the 5th string with your 4th finger, and while still holding that chord shape you add your 2nd finger to the 4th string and 1st finger to the 2nd string creating the C-chord sound from 3X2013 - and then revert to 3X0003, and so on. I am sure you will recognise the first few bars of at least one or two famous songs.

In conclusion - don't give up!