Definitely buy on StubHub. They've had excellent deals on tickets this year. Avoid CUBS.com or even the ticket window's day of game sale. Ironically broker/scalper prices have been cheaper than the Cubs themselves....a number of reasons; dynamic ticket pricing from the Cubs, early season sales with a premium cost, and all the added fee's and taxes you pay with buying direct from the team.
I sit in the LF bleachers with a bunch of regulars, some of the most knowledgeable fans in the park, SABR/historian types but I've sat all around the park over the years for hundreds of games. Most seats are good but I'd try to avoid sitting under the upper deck over hang behind the girders. You can't see fly balls or the center field scoreboard and the girders can obstruct site lines. The first few rows of the upper deck are actually excellent seats and the lower level corners are good too; slightly elevated with a great view. Some sections can get pretty warm on hot days with the sun beating down.
You can bring in something to drink, non-alcoholic, as long as it's in a plastic bottle and factory sealed. You can also bring in snacks and food, also sunscreen as long as it's not aerosol.
An easy walk down Sheffield is Home Run pizza. A good Italian beef place is Al's Beef, a short walk down Clark. The park opens at 11:20. I recommend getting there early, I'm talking before the gates open, they check bags and have metal detectors...it all takes times. It gets really congested around the park as game time nears. I enjoy batting practice when they have it and arriving early allows you to walk around the park and soak up the atmosphere.
The new plaza on the west side of the park is nice place to hang after the game if you want to avoid bars.
A fan site that may have some info.
https://www.bleedcubbieblue.com/ I know the guy that run's it.