Here is a selection of classical guitar strings. If you are new to nylon strings and looking for some advice. I recommend you get the EJ45 D'Addario Pro Arte strings. They have a good sound, good durability and at a good price. They are a good value and give a good performance. If you want to stock up on them this same string comes in a three pack at a discounted price.

If you need to know how to string up your guitar I have a complete tutorial with a video that shows you how to do it step by step here: How to string your classical guitar I also have an interesting tutorial on nylon classical guitars and how they differ from steel string guitars: About the Nylon Classical Guitar

D'Addario Student Strings Set, Clear/Silver Norm		D'Addario EJ45 Pro-Arte Silver/Clear Normal Classical Guitar Strings (3-Pack)
Rotosound CL4 Black N' Silver Classical Guitar Strings		D'Addario Pro-Arte Strings Set, Clear/Silver Hard

If you are absolutely new to the classical guitar, have just purchased one and are unsure about the whole thing you might want to take a look at your guitar. 95% of classical guitars have bridge holes that you pass the strings through and knot them. But some acoustic folk style classical guitars have nylon strings with a ball end on them. This type of strings is made so you push the ball end into the bridge of the guitar then stick a peg in to hold it in place. Take a look at your guitar and compare to the photos I have below.

In this picture you see the six strings passed through holes in the bridge. Then the strings are knotted. For this type of guitar all of the above strings are correct and you can purchase any of them with confidence.

In this picture the strings are plunged down into holes in the bridge and pegged with the white pegs. This is a different type of string.

This picture shows the ball end of a string. This is the kind of string you use on a guitar like the black guitar shown above. You would order ball end strings like the selection I have below.

Classical Guitar Strings Regular Tension, Bronze Wound Ball End