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".