Using good quality strings on your ukulele can make a big difference to the way they sound. New ukuleles often come with low quality strings - which can make them sound disappointing straight out of the box. Changing them for something like Worth or Aquila will improve the sound a great deal.
Buying ukulele strings is a little complicated. Two things you have to check before you buy your strings: the size of ukulele and the tuning they are intended for.
It's very important you check the size of ukulele the strings are made for. Strings will be a different length and thickness depending on what size ukulele they are made for.
You'll also want to double check the tuning they are intended for. In particular, check whether they are low-G or high-G strings. If you are playing a re-entrant ukulele (which is most common ukuleles) the the G string has to be thinner (and therefore higher) than the C-string. If you get the wrong type, you won't be able to tune them the other way. Usually, if the description doesn't state either - they will usually be high-G. If you're not sure, it's worth asking.
You can usually use a low-G set of strings on a high-G ukulele with minor adjustments depending on the type of ukulele.
You don't need to worry so much about whether they are intended for C tuning (GCEA) or D tuning (ADF#B). The tuning are close enough together for it not to matter.
Getting the wrong set of strings happens to everyone. I've done it. But finding yourself with a set of strings that don't fit on any ukulele you have is a great excuse to buy yourself a new ukulele.
The most popular strings are Aquila and Worth. I've tried other makes of strings, but always come back to these two.
Ukulele string buying advice.
If you look around the ukulele forums, you'll see almost everyone recommend either Worth or Aquila. They are the most popular brands by a long way.
You will see professionals recommend other brands. For example, Jake Shimabukuro endorses D'Addario strings and James Hill uses Hilo strings (he also uses D'Addarios for low-A strings).
Something else to consider is that different ukuleles will prefer different strings. Some ukuleles sound better with Aquila strings and some with Worth strings others with Hilo or Kala strings. Trying out both Aquilas and Worths on a ukulele will give you an idea of which suits it best.