The group get to the destiny (the ship) by use of the 9th chevron, which also requires an enormous amount of energy to be dialed, more than is stored in your standard gate. There is a gate on the destiny and will be used to allow the teams to explore planets and find supplies in order to survive on the ship. They can't go home because the ship is automated and they're too far away to dial earth, they'd need alot of extra power to do this, which I assume isn't available to them. There will be shuttles on the destiny but I'm not sure when they'd be used as I guess most travel will be via stargate. Oh yeah one thing I've missed, the people on the destiny weren't ment to be there, they're all at a facility that is researching the 9th chevron and the destiny myth, when it's attacked, they're supposed to dial earth but Dr.Rush sees this as his only chance to get to the destiny, so he dials it, trapping himself and everyone on the destiny.