I read a lot how by the end of the game you become a millionaire.
But I'm level 21 now and I'm struggling with money.

I never had enough money for my items and to learn skills, and now in order to learn each skill I need to pay 10,000 gold !
I do quests and kill monsters but Ievelup faster then I gain money.

The best source of money is just selling the stuff you find. Fill up on spare weapons and armor, load up your horse with loot, then teleport back to town and sell it all. You should get plenty of money.

I never had that problem, in fact it was the exact contrary, since I did exactly what Waltorious describes. It's especially easy when you have the teleportation stone: you can grab everything in a dungeon, go sell, rince and repeat. ^_^

Yes, at the very beginning you struggle a little with money, but as soon as you have this teleportation stone, it's a piece of cake, really.

One of the best ways to improve equipment early is to utilize the stacking feature. I used the Heavy Grom Shield until well past level 20 because there were so darn many of them dropping as loot, I actually got mine up to Class 50 (the max stackability) with a Parry value of 176. This let me save up money for other things I wasn't finding in loot drops. Plus, when you stack items, you reduce the the amount of inventory space needed, and the weight of the improved item doesn't increase. When you do want to sell the item, you will get more money for it if it's been upgraded.

Build reputation with the Brotherhood, Giriza, and the Merchant's Guild early for discounts from their suppliers and/or bonuses to the price they pay you for goods.

You pay 10,000 gold for the trainers in Cathalon, but I paid much less for some skills (300 gold) by picking them up in places further north. Look around the Rusty Oger [sic] camp and the Brotherhood camp just to the east of there, as well as the northwestern villages. There's a list of all trainer locations at GameBanshee.

Only at first is money much of an object. At level 21 you still may not have passed the point where it really starts to come rolling in but it should be coming soon. Buying armor in the beginning can be a bit taxing and its probably best to try and just take what you find off of bandits unless you are trying to use robes and then that is expensive and not too useful.

Try using traps and nail bombs early to lower your need for nice equipment and be willing to sell every herb and stone you find save for boar tusks and bear claws. Most of that stuff ends up piling up and is better used as money early on if it doesn't boost strength.

Some things you do want to save for is training which will be expensive, but once its done its done. Balance is probably the only one you need but it will cost quite a bit.