It’s a really good idea to know which string is which note on your ukulele, this is something that you should commit to memory as soon as you possibly can (even if you’re using a clip-on tuner which I recommend). It will help you a huge amount when it comes to transcribing songs yourself and working out which notes are which.
Ultimately you should be aiming to know every single note on the fretboard but for now get the notes embedded in your brain. Starting at the top the strings are G, C, E and A (A being the closest string to the ground when you’re playing).
Mnemonics To The Rescue
It’s a pretty good idea to use a mnemonic to help you remember those notes – a few that I’ve heard that should do the trick are… Go Crazy Every Afternoon, Giant Cats Eat Aardvarks and Garfield Can Eat Apples. Whatever works for you, it doesn’t matter but it’s definitely worth spending 60 seconds of your time to commit them to memory.