Do buy a manual (if one is not provided with the course). Get hold of it before you start your training and read it. Scribble on it. Highlight bits of it. You won't understand everything in the manual and that's ok. The instructor is there to help and educate you. You'll find if you do your pre-course reading properly you will get much better value from your course and understand things easier in the classroom when big slabs of knowledge start to join up.
Don't be macho when carrying dive kit, ie lifting twinsets. If you need a hand to carry kit, then ask. Everyone needs help sometimes but don't ever play the "girl" card to get guys to carry your kit. Have some self respect and be able to lift and carry stuff. If you struggle ask for tips from other divers how they lift and carry things.
Do watch your instructor when they are teaching you skills, ie "which hand are they using"? You will get more from it.
You don't have to like your instructor. They are not there to become your new best friend. They are there to teach you how to dive safely and efficiently. I hated the guts of the instuctor who did my DiveMaster. He was ex-SBS/Royal Marine and we had a tough course. He pushed us and demanded so much from us. I have thanked mentally so many times over the years because I learnt so much from him.
Do train with other agencies. Become a hybrid diver. You will have a more rounded education, understand the different training ethics and ethos, and as a result you will be a stronger diver.
Don't surface at the back of boats, you don't know when someone is going to turn the props on and they are v. efficient diver mincing machines.
Don't get screwed up when you have that "safe diving accident". You are not the only one it's ever happened to, we all have had them and discovered we are mortal and scared ourselves silly. Learn from it. You will become a safer, wiser diver. Complacency kills.
Don't think that because some divers know more than you, they know everything. They don't. No one does. We are all still learning. It's just that some divers know a bit more than others.
Do read anything you can get your hands on. It doesn't matter if it's another agency manual or something you can't really understand, you will get something from it, as as times passes and your knowledge grows, it's worth revisiting the book because you will get more from it.
Do go diving and don't rush from course to course. Each course teaches you a core set of skills. You need to go diving and consolidate these skills to learn. So get in the water and have some fun. Having a load of badges doesn't necessarily mean you can actually dive.
You don't need to dive deep to have a good dive. Some of the nicest dives I have done have been above 20 metres (approx 60 feet).
It doesn't matter what you see other divers do, always, always, always Do a buddy check. It's your life, respect it.
Do learn the Rule of Thirds and apply it to your diving, so that when Murphy does show up, you have a decent amount of gas to get you out of the mess.
Do carry a Delayed Surface Marker Buoy and reel. It doesn't matter if your buddy has one. You need one too, because one day you will get separated from them.
Some days you just won't want to dive. You don't have to. (Don't let money, travelling etc force you to dive). Have the courage to call a dive at the right time. It's better to be on the boat wishing you were in the water, than the other way around.
If you are going on vacation to learn to dive or do a course, don't go with the mindset you are on holiday. Do go mentally prepared that you are going to do a course in a different country and you are there to learn. You will get more out of your course. If you want to lie on a sunlounger, don't book a diving course.