If you're looking to buy your first ukulele, I recommend a cheap (~$50) soprano from Amazon. It’s a low cost investment, and shipping is convenient. Don’t go too cheap (~$30) or else quality will suffer.
I’ve bought the following 2 ukes from Amazon:
I got a nicer ukulele after playing for about a year. I bought a Kala concert travel ($200) from Guitar Solo. Travel ukuleles are thinner, so it’s easy to pack in luggage. The material is better, and the uke has a crisper, louder sound.
Got a Ukulele has tons of more detailed reviews.
If you’re starting out, there’s no need to worry too much about the particular wood, the brand, or the strings. The differences are subtle.
The biggest difference is size. There’s four main sizes: soprano, concert, tenor, and baritone. There’s also a sopranino, which is smaller than a soprano. Here are some more pictures of different ukulele sizes.
The sizes differ in tone and tuning. Larger ukuleles are louder because there’s more space for sound to resonate.
However, by switching out what strings you use, you can customize the tuning. For a deeper base tone, you could replace the high G string on the soprano with a low G.