Assuming that this is a legitimate question and not some sort of weird spam thread, I would have to say this:
The most important thing for a newbie to learn is the various classes and how their skills function/compliment each other. For example, a necromancer who doesn't know how his curses function or when to use them is not good for much.
Learning how to build an effective character by placing skills wisely is a challenge even for veteran players. But it's good for a newbie to read up on guides and learn the basics of how it's done.
Don't be afraid to ask around for advice and information.
Don't be discouraged if people are rude,- and make fun of you for beeing new. There are alot of rude people in the Diablo community, and you'll most likely meet alot of them. Just don't pay heed to those.
I'd recommend various profession forums on this site, but most of the threads and advices there are for "perfect" character builds. - Those can be hard to follow if youre totally fresh.
Just play along and have fun. Experiment with different skills and different classes. You will make mistakes in skill distribution, and you will make wrong decicions, but we all do that. In time you will find a character that works well for you. Building characters, finding equipement and experimenting with different skills is the true gem of Diablo which makes this game so fun.
People will try to tell you how to play,- but play the way YOU want to play.
My only advice is to be nice to others, and remember that they are "real people" behind their characters. "Do to others what you want them to do to you."
Again, don't feel bad if people are rude to you for making wrong decisions. We've all been new at one point
Use the hotkeys to quickly change attacks and auras. You can use the mouse wheel to quickly change skills.
Once you get the hang of playing the game you should start looking for set and unique items and your charaters will start to shine. Normal Mephisto is a good place to start. Just run him over a few times and you should start to see what I mean. Of course the nice items are going to be found in Hell but you'll have to level up a lot to use them.
Read various guides for all the different classes, then try out a few to see who you like. Don't just believe the first build guide you read...read a few. Learn to use your hotkeys. Peruse the forums, especially the FAQs and other guides people have put together. There is a ridiculous amount of information out there, just read through and have fun. :wave:
The best thing you can do while on B.net is to strive to be kind and polite at every turn. There are far too many punk teenagers on B.net already. If you develop a reputation as a fair and honest player you'll attract the attention of similiar people. They will befriend you and teach you all you could ever want to know. You'll make a solid friend or two with people possibly on the opposite side of the country. You'll eventually collect a pile of gear that you'll be inclined to just toss out. Don't. Give it to a new player. Another friend made. Don't beg-EVER. If you decide you want to be a rude scammer that's the only people you'll ever play with. Watch the language, it's too easy to be a crass loser.
...and of course keep asking questions here.