Honestly, find what what's best for you in terms of your gameplay, because I can tell you right now that if you hate say, playing hobo (good xp job, better triple xp job), then grinding hobo is going to make you wanna blow your brains out. Fuck around with some jobs that have xp categories you like grinding (if you're a cop main, get bounties. If you like shipwreck diver, collect trash and lockers, sell lockers or use them at captain charlie for mad xp), look at the !motd command to see what the xp bonus is at the time, harvest mats. There are a LOT of things you can do to gain xp.
As for the cereal monster, collect any amount of cereal between 2 and 75 (2 is the minimum you need to make a wish, while the most expensive is 75). Once you've dropped them all onto the platform of the cereal monster, press e, and look at the list, and select which wish you'd like to make. Remember that there is a wish to reroll cereal targets, so if you can't find anymore people to get cereal from, just reroll instead of needing to completely relog.
Hope this helps!