There are two basic ukulele tuning mechanisms: friction tuners and geared tuners.
These are pegs screwed through the headstock which hold their place thanks to the friction on screw on wood. They are being used less and less frequently, since they make fine-tuning difficult and often have to be retuned.The only tuning mechanism we can recommend are good geared tuners with bearings. The sole advantage of the friction tuner is the lower weight thanks to its fewer parts which makes the ukulele less top-heavy.
Geared tuners have become standard by now. There are great differences in quality, but they all are much more precise and easy to handle in general, and thus recommended.