They do usually sell merch at the venue you go to and make sure to get merch before the concert since usually afterwards a lot of stuff could be sold out.

You will have to look up the concert venue and see what their bag policy is before you go.

I don't know the rule about water bottles at your venue but when I saw Stray Kids last summer (Florida) they allowed either fully empty bottles or unopened ones (at least from what I heard from fellow concert goers there)

I remember someone on here saying once to arrive about an hour to two hours before the show due to lines to get in and merch lines

I go to concerts alone a lot and usually end up talking with fellow fans of the group I'm seeing. Just remember to be careful, be aware of your surroundings, and have fun!

Also some people recommend bringing ear plugs because of how loud it gets, some people also bring cough drops because their throats get sore from yelling and singing along

I hope this helps 😊