Hey Invalid I’m assuming you’re playing on green? The answer will be wildly different as the pet window has been allowed to remain on blue but does not exist on green.
On green you best bet is to create macros for the main pet actions (/pet attack %t, /pet guard, /pet back, /pet follow, etc). I then add a line under each that makes the pet report his health (I’m playing on blue so I forget the command but it’s something like /pet health or /pet report). You get an update each time you command the pet and if your worried mid combat you can just spam /pet attack to get an update.
For the other items the wiki (which Bodenn linked to but that location didn’t work for me) has tons of great info. There’s a pet guide section and several mage guides. The answers aren’t so clear cut to fit here but will depend on your style, budget, and situation.
https://wiki.project1999.com/Main_Page