don't "go with the flow" - this is a recipe for disaster! you will end up going with his flow and then realise that he's doing nothing, and you're going with the flow.
1) rent - paid together, on time, no questions, no asking.
2) food - assign each your own cupboards and your own shelves in the fridge where you keep your own stuff. if necessary, label things (in case friends/guests come over and raid the kitchen).
3) message board - get a magnetic white board / cork bulletin board. you may need to leave messages for each other while out, or pin receipts and stuff.
4) common items - make a list of stuff you have to share (toilet paper, cleaning supplies, condiments like ketchup, salt/pepper, lightbulbs, paper towels, etc), purchase during your shopping trips, keep the receipts in a box initialled by whoever bought the stuff. at the end of the month, add up and divide the cost.
5) cleaning - together you should develop a task list of housework which needs to be done every week (garbage, recycling) or every 2 weeks (cleaning kitchen sink, refrigerator, bathroom sink, toilet, tub/shower, sweeping, vacuuming, etc.). divide up the tasks, check them off the list when they are done. rotate responsibilities every month.
6) bills - internet, tv, neflix, utilities, and other common expenses. hard to quantify consumption, so you just have to agree what you will split equally, and what you will opt-out and not participate in. after the first month, you should both know how much you each have to pay, so keep a utilities folder/cookiejar/box where you keep your common bills, and figure out who is going to pay each month.
7) guests - boy/girlfriends, buddies, loafers, couch potatos, family members, whatever. establish a maximum for them staying over (e.g. 2 days/nights a week), otherwise you have an additional room-mate who isn't paying for anything. and even though you can't count how much ketchup, toilet paper or internet they are consuming, they are a regular presence and may start grating on your nerves.
pets - probably best not to have anything bigger than goldfish, until you guys get into a rhythm. the scent of urine, kitty litter, shedding, kibble, etc will become annoying, and the pet will be the embodiment of everything you hate in the room-mate (which is not fair to the pet).
9) don't sleep with your room mate!
10) don't steal each others food, toilettries, underwear, boyfriends/girlfriends.
11) make sure you guys go out and party regularly as buddies every few weeks, but also give each other space to be alone/private.