~ Guides and pivots ~
Start by learning the chords G, Em, Am, and D7. Use the fingerings as shown. Once you can make these chords easily, start working on changing the chords using these two techniques-pivots and guides.
The pivot finger is a finger that can stay in the same place when changing chords. From Em to Am, the 2nd finger can remain where it is. This is a pivot finger. This technique cuts down on mistakes and allows notes to ring during the chord change. From Am to D7, the 1st finger may remain where it is, again a pivot finger. The guide finger is a finger that slides along the string from one chord to another. In this case, when changing from D7 to G, the 3rd finger is a guide finger. Leave it on the 1st string and slide up to the 3rd fret, then make the rest of the G chord. From G back to Em there is another pivot finger. The 1st finger pivots form G to Em.
Pivot and Guide fingers will smooth out your playing and cut down on mistakes. Always look for and use both these techniques whenever possible. Not all chord changes will have a pivot or guide finger, but many do.