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I learned this somewhere in class and I can't remember to save my life. I need to make entertain [divertido] into entertaining. Help please?

but you also need to make sure you have the right ending depending on the verb. In the example used above, to study: estudiar, ends in -ar so the -ing ending also has to start with -a so it would be estoy estudiando.
So for -AR verbs the ending is -ando
-ER or -IR verbs would end with -iendo
you also want to make sure that you're only replacing the last -ar -er or -ir ending and not other parts of the word.
divert/ir

although I have to be honest with you I have never heard it put like that. I'm not saying that it's incorrect but idiomatically I don't think anyone says "estoy divertiendo" but I'm not sure. Does anyone know?

Along with all of the above, you need to make sure that you are using it to tell what the person is doing and not to describe the person's personality.

I.E.
estoy divertiendo
soy divertida

The first says that I am physically entertaining (them) right now.
The second says that I am a fun and entertaining person and is describing my personality.

-ing is called "Gerund" Yay for gerunds! I'm not sure what it is in Spanish, but

This is regular Estar:
Está divertido - She's/He's/It's entertaining (state of being.)

This is regular Ser:
Es divertido - It's entertaining (Describing it..like "I'm nice" describing it)

but what YOU want is the Present Progressive. (is it progresivo presento?) It's like "Honey, clean your room!; Mom! I'm doing homework (right now)!

Está divertiendo - She's/He's/It's entertaining (RIGHT NOW. They're entertaining RIGHT NOW. It's in progress.)