A few more things to add:
1. Don't get rushed. As MapleBarb put it, you learn a lot about hc, inherent dangers, and also level appropriately to where you are playing. It's the easiest way to learn to play in HC.
2. If you seek help for some particularly tough end bosses, do it in a private game with people you know - non-ITAMS people who won't grab and dash. The first time you kill an end boss, you get a better drop. Starting out in HC again (i started 2 months ago after 2 years off), i did all my end bosses privately. Even if uniques didnt fall, the rares really help.
3. Collect flawless gems. You can pick up really good equipment for the price of a couple pgems, and great equipment for 20-40 pgems as well (skulders, um, low shako, shaft, occy, stuff like that is all pgem priced).
4. Don't be afraid of pubbies. You'll need to level somehow before progressing between skill difficulties - it'll make it safer for you (more life, better skills). I usually use as a benchmark of lvl 42-45 for moving from normal to NM, and lvl 70-72 for moving from NM - hell. Typically, this is when experience from baals starts to drop. Pubbies (more specifically baals) arent usually a problem in normal. Take notice of unusually high characters, but sometimes they're just there to help level a friend or another character. Learn which characters can tppk (these are the ones to worry about, legit pk's in baals are easy to avoid. Go back to town), and stay away from them. In NM, be a bit more careful. Typically, you need to be aware of everything that is going on around you. If someone starts throwing guided missles out near you, stay far far away from them and leech exp instead. It isnt as quick, but it'll keep you alive.
5. Learn to ESC, Quit Game very quickly. Whether you get hostiled, you are block-locked and in trouble, or you suddenly notice while you're in the middle of a mob that the guy pounding on you is multi-shot, LE, FE, extra strong. Valour is useless at that point. There's no pride in HC. ESC-quit is a great way to get out of trouble.
GL.