Buy a ukulele. You can find them at most of your local music shops. Decent ones are about $76 aus but if you plan on just mucking around the cheapest are about $25 and they work fine for beginners.
Next you have to tune your ukulele. You can either buy a tuner or if you are already pretty musical tune it by ear to the notes: g c e a. I use a ukulele tuner app. It cost 99 c but its well with it.
A new ukulele won't hold it's tune very well the first couple of times you play it. It will slip out of tune quite quickly. So just stretch the strings or strum while you watch tv to make it better.
The most common strumming pattern to use is down. down. up... up down. Try practising this without chords to get the hang of it
Decide whether you prefer strumming or using a pick. If you want to test which you prefer without buying a pick try using a paper clip to get the general feeling of how to use a pick.
Next learn some chords. You can find how to play chords on many websites or on apps. The most commonly used chords are c, g, d and f.
Next practise changing between chords. Sit with your ukulele and strum c chord for 4 counts then change as fast as you can to g. Continue changing back and forth. When you start this will be shaky.
Keep practising and improving :) thanks for reading