In reply to JamButty:
I haven't ridden at all for about 10 years so my knowledge might be out of date, but when I learned everyone learned on a 20". 24" was just if you wanted to go faster or further.
You won't notice the difference in quality until you start doing tricks or jumping around.
If you get a cheep one, you will bend the crank when you start bunnyhopping around (probably your first trick). I would get a chrome one rather than a coloured one, as it won't show up the scuffs so much.
If you want something cheep, these look fine - they have the advantage of a square frame, rather than a rounded frame, so you have somewhere to rest your foot when you are riding one footed - again quite an early trick.
https://www.unicycle.uk.com/unicycles/adult-learner/club-unicycles.html
If you want to spend a bit more and get something you can jump around on, then these will be great.
https://www.unicycle.uk.com/unicycles/adult-learner/nimbus-unicycles.html