Keep it simple -- be honest about your skills and willingness to learn. It's great learning on a job but you need honest communications with your new team. Make sure you know enough (or trust you're learning process enough) to give the company value or not get in over your head for something mission critical.
Once you've gone through the interview process and they've hired you, trust them! Don't second guess yourself. At that point, tips are:
* Be prepared to spend time outside of the job getting up to standards on the code base or things you don't understand, especially the first few months. You want to make sure you can also deliver what you've contracted for.
* Are you willing and comfortable asking for help from other people? If you have good sources to go to and you go to them frequently while you're learning, that'll help a ton.
Anecdote: I've seen folks (it doesn't sound like you're one of them though, tho) who kind of hold their breath and hope folks don't notice what they don't know. That's toxic: bad for the company and for the person (it stops them from being able to learn b/c they're so afraid to admit they don't know.
As long as you trust you can learn, work hard, and deliver -- go for it! There are always going to be new things to learn and jobs that are "just beyond us". It's finding the line between "way over your head" and "a good stretch" that's important.
A recruiter contacted me via email looking for a developer "at the Jedi level". Interestingly, though she said she'd read my resume, she was looking for a Java developer in Maryland (I am a Ruby developer in Chicago). I asked her to clarify what skills the Jedi level entailed (I couldn't help myself). She wrote back and said that it meant they wanted "a rock star"