I'm in need of a strawberry filling for vanilla cupcakes. I also need to know how to fill them. Please help!!

I use jam, or jam mixed with a bit of buttercream.

I put a large tip on the bag(with an adaptor just in case) and poke a bit of the filling in. I don't worry too much if it comes back out of the hole, or it can slightly crack the top. After one or two, you will be able to feel the cake get a bit heavier. smooth the tops as needed and Cover the hole with your icing