Choosing the right type of strings

Not any type of strings can be put on any guitar.
Steel-stringed guitars have iron bar in the neck, which prevents bending of the neck. This is because steel strings have more power when strung, so a classical guitar's neck wouldn't survive long if steel strings are on it. Although some guitars without iron bar can have steel strings, but they must be thinner, commonly 0.10 are used. Most common sizes of strings are 0.10, 0.12, 0.13, 0.15 and 0.16. The smaller the size, the easier is to fret the strings. Sound is different at every size. For beginners, best choice are 0.10 strings.

Nylon strings are used with classical guitars, as I said before. They are used on guitars that doesn't have strong necks, because they can't bend them. Well, they can, if the neck is made out of paper...