Hey, I'm a newbie, it would be very useful Like, the bags, what's inside if I buy Anyone can help?

I don't know of a singluar guide, but you can look the bags up on Subetalodge to see a list of items in the bag. The bags under Chance will give you a random item out of the possible selection, and those under Choose will let you choose a single item. Costume Trunks will give you the full set of either male or female items (you can see those and also Boutique items by clicking on the set in the Cash Shop though, which is a bit of an easier way to look at wearables)

Monthly Collections are released usually on the first or second of each month, so if it's easier for you to look at them with the item hovers, you can use the News Archive to open the news page for a certain month and they should be near the top of the page (although I just noticed that ALL past collections were brought back so uh, that would be a lot to check, and if you go back far enough, not all the items are listed in the initial news post. But recent ones are, at least)

here is a thread guide about the collections! (it might be slow to load as it's very image heavy!)

cc :)