1) 2 minerals per gatherer (which gives you 16 SCVs per line), BUT keep making SCVs till you have 20-24. Saturation is at 24 SCVs (3 per patch) so you'll benefit from extra SCVs but at a reduced rate. Anyway, you transfer anything extra of 16 to your expansion once it's done. One of the most important things to learn as a newbie is to make SCVs constantly. Don't queue more than 2 per CC.

2) Study a build order and build your refineries according to it. Fit 3 SCVs per refinery.

3) Hotkeys are really self-preference. There are different ways you can key your bioball; Hotkeying marines and marus into separate groups lets you focus fire effectively for eg maru on stalkers while marines on immortals. Hotkeying a mix of marines and marus into 2/3 groups of about same mix and size lets you make a good concave on the move which will be good when fighting hydra/roach armies for eg.

Anyway this game is intense and you are definitely not alone when it comes to raging after a match. I usually have to take a break after 2/3 games due to the crazy stress level.