Aashley333 gives some great directions and suggestions.
You should know that many of us don't like to do t-shirt quilts for a number of reasons. It may not be the best introduction to quilting but you are doing it with love and that makes a big difference
I prefer to border each t-shirt block with woven cotton quilting fabric and make consistent sized blocks. Not near as interesting as Aashley's "jigsaw" design but easier for my mind to comprehend and for me lets work with more variation in the t-shirts colors and shapes and such.
Be aware that there are many ways to go wrong and it happens! Be careful with the interfacing to avoid melting it to your iron! There are ways to get it off. Best of luck to you -- we are here for when things are not going well or to share the joy of a successful project.