Choose the right WOOD:
Koa has that classic Hawaiian sound. It's very mellow and clear, and the grain can be spectacular. It's also more expensive.
Mango and Curly Koa have a lot of fancy grain and shimmer. They get expensive, too.
Mahogany is a softer wood, and has a softer tone. It's more affordable than Koa, but it scratches more easily.
Laminate is not a tonewood. It is several layers of plywood laminated together, and sometimes topped with a thin veneer of mahagony or other tonewood. It's the most affordable.
Myrtlewood is an unusual wood for ukuleles, but it can have a stunning grain. It sounds bright and resonant.
Choose the right STRINGS:
Your ukulele will have an entirely different sound when you play strings from different makers. This is an overlooked but MAJOR thing to consider. Wound strings are nylon strings that are wrapped in a layer of metal or nylon. They are loud and bright, but they also create a lot of noise when you slide your fingers along them while playing. Monofilament strings are nylon strings that are not wound.
LOW-G vs. HIGH-G --- Low g-strings are a lower octave and are therefore deeper-sounding. High g-strings are a higher octave, and thus the "bass note" has a very high pitch. If you are a guitar player, you'll have to get used to this. Low g-strings are usually wound strings, but you can find monofilament low g-strings if you want to avoid string noise.
Kala Red strings are very bright and have a long resonance. They are colored bright red.
Worth Strings are also very resonant. They have monofilament low g-strings that sound really nice. They are colored brown.
Guadalupe Custom Strings will custom-make any strings you want, and color them in any color. Any instrument, including European or Asian instruments.
Choose the right SIZE:
Concert: Slightly bigger than the soprano, and sounds slightly deeper. Still small and sweet, though.
Baritone: Biggest. Tuned like a guitar (D-G-B-E).