I want to watch Insidious: Chapter 2. But do I have to watch the first part or is this a stand alone movie?
"Insidious: Chapter 2" is a direct sequel of the 2011 movie Insidious.
Wikipedia at help.
The film received the title of Insidious: Chapter 2 because it is a direct continuation of the first installment. However, the tone of the film was to be more grounded than in the first film, with Wan citing his work on The Conjuring as an influence to how he and Whannell approached Insidious: Chapter 2. "I pulled things from Insidious that I applied to The Conjuring, and what I learned from The Conjuring I applied to Insidious 2," said Wan. "So for me, I feel like it’s a cumulative filmmaking experience that I’ve gathered over the years."
I haven't seen the Chapter 2 yet but I have seen the first movie and trailers for the second movie which appeared to be a direct continuation from the first movie and trailers also seemed to show more light into the origins of the some of the events shown in the first movie.
I would advise you to watch the first before the second, so you're not feeling stumped watching it.