Ahhhhh! Crochet was my first love and I've been crocheting, for over 50 years.
For granny squares, I would suggest 'worsted weight' yarn and
hooks of either H or I. A size G will give a smaller, tighter stitch and J will give a larger, loose stitch than I personally prefer for granny squares.
Yarn is a bit tricky, as not every 'worsted weight' yarn is of the same weight or thickness. Red Heart (sold at WalMart, among many other places) is nice yarn SOMETIMES, but it really varies, from skein to skein. The variegated colors seem to be much thinner and even among the solids, some are rough and some are silky smooth. You just have to look and feel for skeins that are 'plump' and have a bit of give to them.
My favorite worsted weight yarn is 'I Love This Yarn,' sold at Hobby Lobby. It's plumper and works up very nicely.
Never skimp on crochet hooks, when buying them. Susan Bates or Boye are good brands. Many crocheters have a preference because there are slight differences, in the way they're formed. Don't be lured into buying cheap, off-brand hooks, as some tend to have barbs on them, which will catch the yarn and cause grief.