Congratulations on your new pup and good on you for rescuing a pup.
I've always found the best way to introduce a new pup is to have the older dog on a lead and holding the pup in your arms let them sniff each other. Then I put the pup on the ground and holding the lead of the older dog let the pup wander around to explore the new environment. Once you're comfortable and you think the dogs are comfortable you can take the older dog off lead and keep a close eye on how they interact.
It's always a bit of a nervous time introducing a new dog or pup but done slowly it'll go OK.
Another thing to remember is to make sure you don't show the pup any preferential treatment. It's important for your older dog to realise that his place in your pack is still solid and he's not being knocked down a peg with the introduction of the youngster. Make sure you still spend quality one on one time with your older dog, and small things like feeding your older dog before the new one will help him understand that the introduction of the new pup is nothing to be worried about.
Good luck and I hope everything works out OK. There will probably be a few little scraps and tiffs as they sort out their spot in the pack, but I'm sure everything will work out fine.