You need at least a repair kit (spare tube, mini pump, tire levers) at the very least so you won't get stranded or have to walk home (go on youtube to learn how to replace your tube). Some people also brings patch kits. It's a preference thing and I personally don't carry one. To carry your kit, you also will need a saddle bag. You can buy cages (assuming the bike didn't come with one) for cheap if you just need one. Same thing for bottles. You can get those from Wally World. If you want cages and bottles that look better, go back to your LBS. Helmet will cost you $30 or more. The recreational ones are around $30 and road ones are 50 and higher. Again, if you just need a helmet, go to Wally World or Target. Finally bibs and jersey are all for comfort.

I didn't buy any lights just because I will never ride in the dark.