The very first thing you need to know is how to stretch your strings. When your ukulele arrives the strings will be lax and baggy; this is so your springs don't snap in transit. This means that you'll have to tighten up your ukulele and tune it yourself. However a new ukulele's strings will slip out of tune regularly for the first week because they're springy, and so stretching your ukulele will make it far more playable.
You're going to need is an app called Ukulele Tuner. You can of course tune your ukulele by ear, or by a piano but this app is perfect. You simply need to play your string and then it tells you when your string is in tune.
So what you need to do is tighten the pegs on your ukulele and keep playing the string. Along the bottom edge it will say the number that you need to match your string up to...it's quite a hard thing to explain. Essentially when your string is perfectly tuned the line goes blue! Yay!
So you have a tuned ukulele! However, after a few strums it will slip out of tune. This is totally normal for a new ukulele and so you will need to stretch your strings. Basically slip two fingers under your strings and then push your thumb down on the string. Do this in several places on the string and then re-tune. Do this about four times. Each time you stretch your string it should go less and less out of tune until it's basically staying in tune. Leave it a couple of days, keep playing, keep tuning and then if after a couple of days your uke is still slipping have another stretching session.