“It's a lot of fun.”
You're talking to someone who you just met, and you told her that you used to live in New York. She asks what it's like living there. You say this because you really liked it.
It's a lot of fun.
When you're talking about "fun", don't use "very" and "not very". Instead, think of "fun" as having an amount, like water. Use "a lot of fun", "not much fun", "so much fun", "not any fun", etc.:
I had so much fun last night.
It's a lot of fun studying learning to speak to people in a foreign language.
I remember Disney World not being very much fun because we had to stand in such long lines for everything.