“Nothing in particular.”
You're at a clothing store, just looking around. A salesperson asks if you're looking for a certain size or style. You're not, so this is your reply.
Nothing in particular.
Use "nothing in particular" when someone asks for your opinion, but you don't really have one:
A: Do you have any questions?
B: Nothing in particular.
We use this phrase because there's a difference between not wanting anything (like when you don't want anything to drink) and not wanting anything in particular (like when you do want something to drink, but you don't have a strong idea about which drink you want)
"In particular" means "especially":
Class C in particular seemed to be really excited about it.
Class C especially seemed to be really excited about it.
There are a few differences, though. You can't use "in particular" before an adjective, but you can use "especially":
You look especially nice today.
"In particular" usually comes after the thing that it describes:
The Editing Department in particular seemed really well-run.