as far as I know, the tank will come out from underneath the car..
Start with removing the plastic wheel arch covers, you should get a
better view of things..
My guess is there are just a few bolts, an electrical connection,
and a pipe to pull off, but it will be tricky to get to.

As the tank is plastic, you may be able to use an epoxy type glue or
melt the plastic to seal it.. It could be a crack rather than a
hole, in which case I would replace the tank, as it will only get
worse.

John