Ok, so you're 12 and you've got all of these feeling and desires inside you - it must feel a bit scary! But I can tell you - it's perfectly normal and a part of growing up. I seem to remember my own boys going through this sort of thing.
You say your parents are 'very strict on naked people'. Well I think I was too - we didn't walk about naked at home and I didn't exactly encourage my kids to go looking for or at naked people.
Just because your parents are strict doesn't mean your mom won't understand your feelings. Looking at and touching a woman's boobs is something that's common to men - at the right time and in the right relationship! But it's too soon for you.
I don't think you'll get in trouble with your mom. She might be a little surprised (even shocked) if you ask her straight out - because to her you are still a young boy. But you could try.
I think the idea of speaking to your grandma is good. Grandmas are older and usually wiser and - believe me they are hard to shock! Just pick your time and tell her you want to ask her a personal or private question - that'll give her a warning about what you are about to say! Then just tell her how you've been feeling lately. I'm sure she'll listen and won't judge you.
She'll likely tell you everything I've said here - it's natural and normal and your time will come in a few years. Like I say, with the right woman, in the right place and at the right time.