Go and grab any 6-string guitar now and put a capo on the 5th fret. If you ignore the two lowest strings, you have the same tuning as a standard ukulele.
A guitar is tuned EADGBE and with a capo on the 5th it will be 5 semitones higher - ADGCEA - so ignoring the lowest A and D, you're left with GCEA like so:
On a ukulele, the tuning is not linear (not from lowest to highest). The lowest tuned string is actually the C. The G you now have on your guitar will be tuned an octave higher on a ukulele. But fear not, because it's the same note, it has no effect on the chord shapes you use for the guitar.
Now go ahead and strum some chords you know on just those four strings. Sounds alright, doesn't it? Just a bit higher pitched than your used to. All ukulele chords can be derived from most guitar chords, the only difference being that each chord shape now represents a chord 5 semitones higher. For example, If you play a 'C' chord shape on your guitar with a capo on the 5th fret, It won't make a C chord sound, it will sound like an F because it's 5 semitones higher (C - C# - D - D# - E - F). Therefore, if we ignore those two low strings again to get the ukulele tuning, then the F chord for the ukulele will look like this:
This principle can be applied to any guitar chord so here are a few more examples to get you going:
There are some chords that can be modified for the ukulele to get a better sound out of them. A good example is the A7 chord (D7 on ukulele).
You've probably seen A7 played like this on the guitar:
However, when the root note is removed when playing just the 4 strings of a ukulele, there is no A note in the chord. The other A note in the A chord disappears when we play A7 so we have an A7 chord with no A note (or D7 if you still have the capo on). To fix this problem, we make a new chord shape for A7 by putting the G note in the A7 on the highest string like so:
This new chord shape can be played as a bar chord on the ukulele and so you can move the shape up and down the fretboard to get other chords.
Obviously, your guitar is not a ukulele. It still sounds like a guitar and lacks the twangy, comical sound of the ukulele. But, you can now practice the ukulele without having to buy one and then you can decide if it's right for you. If you go and find a ukulele chord chart in a book or online, see how many of the chord shapes you already recognise. You may even find new chord shapes for the guitar in the process!