I always had a problem starting off. How did you get your first items of value?
It helps to have friends, but as Rule outlined, it's all about trading. Nothing in d2 is more important. If you're not good at trading, you'll never make a decent character, no matter how much time you spend. After a ladder reset, you'll find that the characters at the top of the ladder aren't really spending all that much time leveling; they're spending most of their time in channels or on d2jsp trading.
If you haven't already heard of d2jsp, go to d2jsp.org and make an account - every self respecting d2 player trades there. It's a forum geared towards trading - think ebay for d2 items, with forum topics instead of auctions, and forum gold (or fg) instead of dollars. You can buy fg (1fg=$0.035), but that actually would take the fun out of the game.
You'll have to have a good understanding of what items people want - this means you'll have to be familiar with all of the different character builds. I actually found a charm yesterday that I almost sold to merchants, but in passing (I was saying "damn another crappy charm" or something like that), someone offered me some mid runes for it. After doing some investigation, I realized that it was an excellent charm for a barbarian. I put it up on d2jsp.org, and it sold for 85fg. To put that in perspective, SOJs are currently ~450fg (ladder reset about 2 weeks ago).
I think it's pretty sad that the vast majority of the people who play d2, even the ones who are pretty good at it, are incredibly inept at trading. This is partially due to a lack of understanding of game dynamics, but primarily due to the false perception that economics make the game less fun. Two of my friends from BNU basically ganged up on me for trading on d2jsp, primarily taking stabs to the end that it's lame to sell non-ladder items in exchange for ladder items. There's nothing I can say to convince someone otherwise, because it's a mindset issue; if you actually tried to sell NL items for ladder items, you'd realize that you'd make more FG just playing your ladder character and selling ladder items, since NL is already flooded with way more items for sale than people want to buy. A high rune on NL runs for about 15-20fg. On ladder, you can expect to pay 50-100x more with the current market state. I've been selling low runes for 5fg a piece.
So, back to the question: how do you get started. I started by doing countess runs in normal. Lots and lots of countess runs. If you don't already know this, the countess usually drops runes. Low runes. With the market in its current state, I can sell the medium-low runes for 5fg. That's how I got the gear I needed to survive in NM. When I got to NM, I started doing countess runs in NM, getting (slightly) higher runes, and repeated the process, buying better gear and selling the old gear. By the time you get through NM, you should be doing Mephisto runs, and have a bit of MF. The stuff you'll find off of him will sometimes sell pretty well. Making the jump to hell is pretty difficult; being able to solo in hell is impossible without lots of +resist gear.
One of the ways I made a bunch of fg is by making spirit shields/swords. The runeword consists of all low runes, which you can easily find in nightmare countess runs. You can find the 4 socket gear you need in cow runs. A crappy spirit sword runs for 15-20fg; a perfect roll could run as high as 200fg.
