”Picky” is an informal word for someone who is excessively meticulous; a person who is not easily satisfied and choosing (taking a ”pick”) might become difficult for them due to the requirements they have. One can be picky about pretty much anything, but most often I've heard people use the word when discussing food or dating. I'm not picky is added at the end of a sentence to imply that the speaker isn't very choosy and could be content with different solutions, even though he might have an opinion on the matter or a personal preference. ”I can't believe he still doesn't have a girlfriend! I myself have always been with someone. I'm not too picky.” ”I love brunettes. But blondes are okay, too. I'm not picky.” ”Would you rather wear a red or a blue tie? -Either one, I'm not picky.”

Correction: In which context or situations should I use "I'm not a picky"? I'm not picky= I don't criticise, I am happy with what I have

It's usually "I'm not picky". "Pick" as in "choose/select". You use this to mean you are not fussy and you're happy to accept whatever options are given. A: What should we do tonight? Go clubbing? Cinema? Go to dinner? B: Anything's fine. I'm not picky. "I'm not a picky eater" is specifically for choosing food.

I'm not picky = I'm not selective = I'm not choosy I'm not picky about what type of clothes I will wear. I'm not picky about what type of dressing to put on my salad. etc.

many thanks for both replies, it has been very useful to me