Even then I would approach it with confidence. You can learn things very quickly by doing, especially when highly motivated and especially if you can budget some extra time (perhaps unbilled?) to account for the learning curve.

I would only turn down work for fear that you are not capable if you know it is truly beyond your ability for the forseeable future or if the deadline is too constrained to permit yourself to recover from mistakes or delays from learning.

Then you get more pressure. You want fit where you are, don't want to let anybody down. And from the inside you get second breathing and try try try, and one day you see that you are far away from your start.

You can always ask your questions online (StackOverflow and other forums).

People are always ready to help.