Christians: how do you justify belief in the afterlife?

how can you possibly believe in heaven. it just sounds soooo idiotic and childish. "good people go to heaven and bad people go to hell" that sounds like something you tell your kids so they be good. basically a santa clause for adults except being good gets you eternal bliss rather than a new toy truck. if there are all these people in heaven and there can be no bad done, if there is free will than how can all these people avoid sinning? where do all these people live? are there houses? water and electricity? it just sounds so ridiculous to me. imagine living forever, i cant fathom that at all. FOREVER. that means in a quadrillion years ull have another quadrillion set of quadrillion years. catch my drift? it sounds ludicrous. christians only please, i dont need reassurance i want answers