These words are called "determinantes indefinidos" in Spanish, because they talk about an unspecified quantity and they are frequently used in questions. The correct form of these "determinantes" are:
"algún/alguno - alguna" > SINGULAR
"algunos - algunas" > PLURAL
Regarding the use of "ALGÚN/ALGUNO",
ALGÚN is always followed by a noun. ALGUNO can't be followed by a noun.
·If it is before the noun, we use: "¿Tienes ALGÚN libro de aventuras?"
- The common answer is "Sí, tengo uno" or "No tengo ninguno"
·After the noun we use the form "alguno": "Necesito un libro de aventuras, ¿tienes ALGUNO?"
- We can use the common answers above or also "Creo que tengo ALGUNO"
[The same goes for "NINGÚN/NINGUNO"]
*Keep in mind that the rest of "determinantes" ALGUNA, ALGUNAS, and ALGUNOS have the same form before the noun and isolated.
I hope your doubts are solved! ;)