On page one, you learned to tune your G string using one of the little buttons that make up the UkeSchool ukulele tuner, which is conveniently located at the top of each and every page here at UkeSchool.com.
Got your G string all sorted out? Do the same thing to other three. No big rush, get used to how to make the string higher, how to make it lower, and how to ease it gently UP into exactly the pitch you're aiming for. Remember, if you leave your mouse over the tuning button after you press it, the note will continue to play, so you can explore at a comfortable pace until you have the hang of it. Tune your C, your E, and finally your A.
Then go back and check - if you tuned the strings very much, they may have pulled on the neck a bit - and then the strings you tuned before will be too loose! You may have to go through all the strings a few times, to get them to stabilize. On brand new, especially cheap ukuleles, you may have to repeat the procedure over and over for a few days until the strings "settle in".
Try not to get frustrated, especially at the start. Eventually, you will be able to tune your ukulele quickly, efficiently, correctly, and out in the real world where there is no uke tuner. And, as your uke and it's strings settle in, you will have to re-tune it much less often.
Next, we're going to learn how to hold a ukulele and strum a chord, and then you'll be able to check and see if it really is in tune!