1) how do I create my own Dodo code? Dodo codes are used for when you want friends to visit you or you visit friends, they are a one time use. Myself and another member here experimented a bit last night with this. But to create it you go to the airport, speak to the Dodo behind the counter and say you want friends to visit you, then pick the Dodo code option. Then share that Dodo code with another friend. They then visit their airport and say they want to visit a friend and enter the Dodo code.
2) how can I join random islands, I see people saying join and go and ransack for new goods but the airport dude just says to try again another time? The random islands being mentioned are when you spend 2000 Nook Miles on a "Nook Miles Ticket" and go to the Airport, say you wanna fly and redeem the ticket. They take you to a random deserted island which gives you potential access to fruits not native to your island, resources, and also possibly other villagers to invite to your island (NPC)
3) can you build a storage box or something for your house or tent (house just being built), I find my inventory gets mega full quick - I hear you unlock a larger inventory to carry but wonder if something a bit larger to store at my gaff. As mentioned, once your house is built you'll get a "Storage" as part of it. You can dump stuff in there