Tuning your ukulele is the most important thing you will need to learn. There are different tunings for ukuleles but the most common is G-C-E-A.
Here is a website to get you started with tuning. http://ukebuddy.com/ukulele-tuner
If you don’t already own a tuner get one. They can be purchased at most music store and of course online and run around $10.00. Featured is a Snark tuner but there are many different brands to chose from.
Smart phones are also a great source for all kinds of music applications including tuners and they are usually free.
No one wants to hear you play out of tune!