The verse of this Nirvana song provides great fodder for practicing the really basic progression of E minor to G major. The "chorus" contains some slightly more difficult chords - barred chord shapes like C#major and F#major - as well as some quicker chord changes, but none of this should be too difficult with a little practice.
The strumming for "About a Girl" is somewhat unstructured - Kurt starts the song by strumming a "down, down up down up down up" pattern, followed by three bars of down up down up down up down up. This isn't a pattern Kobain sticks to throughout the song though - you should feel free to treat the strumming pattern as a guideline rather than as a rule.
Learning "Smells Like Teen Spirit" allows guitarists to practice three separate techniques - playing basic power chords, playing strumming patterns using muted strings, and the technique of turning on a distortion pedal in mid-song.
The chords for "Smells Like Teen Spirit" are easy - once you've conquered power chords, you're set. Where you'll need to focus is on the strumming. Use your fretting hand to deaden the strings, by lightly laying it flat across the fretboard, making sure it is touching all six strings. Strum up and down while doing this - notice that it can provide a very percussive sound. You'll need to then practice forming the barre chord, playing it, then immediately deadening the strings using the technique described here.
"All Apologies" mixes basic single-note patterns with one-finger power chords. This is a nice easy song, which even the most beginner guitarists can make sound good quickly.
When playing the single note theme to the song, you can use all down-strokes, hitting the low E string when specified to fill out the bottom end. You can get away with playing the open sixth string more than is specified in the tab - it will sound good wherever you play it - but be careful not to strike the open sixth string too hard. The "main" notes of the theme are played on the fifth and fourth strings - make sure those are the notes which ring out most clearly.
In true Nirvana style, the song is tuned down a half-step, but Kurt has additionally tuned down his low E string another full step, giving us the opportunity to use play this low E string throughout the single note theme.
This is another nice, easy Nirvana song that uses both a single-note theme, plus other simple chords. The main theme is played on the low strings of the guitar, and is straightforward from a technique standpoint. The rhythm of this riff may be tricky at first, so examine the tab in detail, and listen to the song to get it exact. By the way, Kurt Cobain uses an Electro-Harmonix Small Clone chord pedal on this song to achieve the wavering guitar tone heard on the recording.
Once you've nailed the theme, and the basic chords, try taking a turn learning the solo - it's a simple restatement of the melody, so it shouldn't be difficult to master.
In the typical fashion, Nirvana plays "Come As You Are" dropped a tone.