A great way to expand your guitar playing is by exploring different tunings, and you can save a lot of time by working on the songs of Joni.
Some of my favourite guitarists are singer/songwriters like Joni Mitchell and James Taylor - lots of acoustic guitar with great chords. Shawn Colvin is another great writer, with an amazing voice and a strong and solid guitar style. This song, Polaroids, is one of my all-time favourites, along with Ricochet in Time. Her cover versions of other peoples' songs are also very good, and these are on her CD Cover Girl.
This song is in drop-D tuning - same as normal tuning, but with the 6th string, low E, taken down to D. You can use open string 4 as a reference note for tuning. This opens up the sound for a song in the key of D, and is widely used in many songs by John Martyn, James Taylor and many others.
More details and video on my other hub, Guitar in Dropped D tuning.
There is also a capo, at fret 3. I've shown some alternative chords (follow the arrows) as they have much the same sound, but are a lot easier to play. Do not hurt yourself playing these chord shapes! The groove of this song is what makes it special. The final chord of the chorus part is shown as A7 sus4, but it's probably A add9 instead. As it's difficult to get to this chord quickly, I'd use whatever you find easiest.
Working out songs - this used to be much more difficult, but using Youtube and looking for guitar tabs online will really speed things up. If you look closely, you can often get a good idea of how to play guitar songs from concert footage - understanding a little music theory such as harmonised scales will also help.
Using Drop D tuning
Just remember that all the standard chord shapes will work in this tuning, as long as you compensate by moving the note on string 6 up 2 frets. Most songs in the key of D can be improved with this tuning, for instance I'll use it to play Norwegian Wood by The Beatles, Dear Prudence, even Here Comes The Sun. For solo acoustic guitar it can really fill out the sound.
The key of D is generally one of the most guitar-friendly keys.
The harmonised scale in D is as follows:
D, Em, F sharp m, G, A, Bm, C sharp m7b5, D
You should memorise this sequence of chords as they are the basis of all songs. As the root note of a D chord is quite high on the open 4th string, adding the open 6th string will give you a more powerful sound.