Interesting is an adjective. Interest is like a noun. We can never put -ing unto it. I think, the first thing you should do is to identify words whether it is a verb, adjective etc.

Thanks Michael! Can you explane me with examples when or how to use "to swim" (verb) or "swimming" (noun)?

You've picked an irregular example my friend! Interesting is actually an adjective, never a verb. A person can be doing the action of running, swimming, sleeping, but interesting is not a verb, if someone or something 'is interesting' it means that that is an object or person in which one might be interested! I think your grammar books might answer your question better than native speakers, as native speakers don't often break this down in their heads, good luck!