This is going to be fun.

This is a phrase that you say when you're going to do something fun and exciting:

This is going to be a lot of fun!

I should explain the pronunciation of "going to". If you clearly pronounce each word, it will sound a little strange. The way people usually pronounce "going to" sounds like "gunna". Sometimes you can see this written as "gonna" in informal writing, like on Internet forums or social media sites. But there are also people who find this annoying, because the formal way to write this is "going to".

this

Use "this" when you're talking about a situation that:

• is happening right now

  This is an awesome show!

• is about to happen soon in just a few minutes
• you've just made plans for or just decided to do, like in the example at the top.

(something) is so much fun

The phrase "so much fun" means "a lot of fun". But "so much ___" actually sounds like a larger amount than "a lot of ___"

When you pronounce this phrase, put the strongest stress on the word "so":

This is going to be so much fun!

I sometimes hear English learners saying something is "so fun", but "so much fun" is better.