Renesmee and Jacob will have a relationship as soon as Renesmee is fully grown. How will their relationship work with the fact that Renesmee will never age further and live forever but Jacob will grow old and die since he is a werewolf?

As a werewolf, Jacob also has the capacity to stop himself from ageing. Werewolves don't age while they're still shifting, so all he'd have to do is keep going into wolf form regularly. The other werewolves just eventually chose to stop in order to age with their families, friends and imprintees/partners.

Technically, Jacob can choose when he wants to stop being a werewolf and will then start aging again. But I don't think he ever will so that he can have his true love with him forever.