I certainly screwed up with my first ukulele purchase. I went in to my local music shop, picked up the only ukulele they sold and bought it without even trying it out. It was a no-name, badly-made hunk of junk. I hated playing it and it spent most of its time under a desk breeding a small but genetically diverse warren of dust bunnies. It wasn't until years later, after buying a decent ukulele, that I really got into playing. I entirely blame that decision for the fact that I am not Jake Shimabukuro. To help you avoid that fate, here are a few tips.
There are four main sizes of ukulele. In order of increasing size they are: soprano, concert, tenor and baritone.
As a beginner, I'd steer you away from the baritone. They're tuned differently from the others and it's much harder to find tabs, tutorials etc.
Of the other sizes, soprano is the traditional ukulele size and the one most often used by beginners. With it being the smallest, there's less stretching for notes. They also tend to be the cheapest. Overall, I'd recommend starting with a soprano.
Concert and tenor sizes will give you more room to maneuver on the fretboard. Tenor is the size used by most professional players these days. Both concert and tenor will work well for new ukers. You play them exactly the same as
How Much to Spend?
You can get some very cheap ukuleles but I'd recommend spending at least $60. If you buy a very cheap ukulele, it's very likely to have problems which can put you off playing.
Equally, I wouldn't recommend blowing hundreds and hundreds on your first uke. Save up for when you've played a while and have a better idea of what you want.
There's a really good selection of ukuleles for beginners these days. There are also a few duds. For your first uke, I'd recommend steering away from anything too crazily shaped or highly decorated.