Levels 1-20 should be very simple.
First thing you should know about is to visit every maiden on every map you come across. Cash shop currency is purchasable ingame, so later on it will be completely feasible to buy premium days, this will let you teleport between maidens at will.
Second thing is to try to avoid selling your 'loot'. Stash everything in your warehouse which you can access at maidens. Once again you will need cash shop currency to expand your warehouse and bag slots.
Third, don't worry about purchasing armor and equipment. There are loads of quests up through the early 20s, many of which will provide equipment and when you finish your lvl 20 class change quest you will get a new set of crusader or dark knight gear. When you get new equipment from a quest examine it carefully, it might not be obvious but when you get a quest that rewards you with gear you already have it will normally have a card slot you can attach things too - not that you need it.
OK, so basically, just do quests. As a warrior it is easy. Dump your points into stamina, when you get to a 10s, dump points into strength. When strength is at the 10s and caught up, dump 20 more points into stamina, and so on. This will give you the ability to take hits and do damage. If you don't like this, you will get a free reset usable up to lvl 20 in the form of a quest in the capital.
When you get a quest for the 'elenium mines' save the quest and put it off until you have a bunch of them and closer to lvl 17 or 18. The elenium mines are under the windmill in elegore and is a PVP zone. I recommend the mario hat and the lance recipes for one, the mario hat is better than the hat you get from the free set and is usable at a lower level and you don't get a lance in the free lvl 20 set.
Your biggest obstacle will be the class change quest at level 20. I went crusader but from what I read the dark knight quest is the same. You have to fight a monster. Clear out the dungeon before the monster spawns - bring fast acting food like fried rats and sugar. (EDIT: DO NOT USE FIRST AID! Press K and add "rest" to your hotbar. It is just as good as first aid but does not use energy!) Bring one 4k heal potion and one 220 energy heal potion. Make sure Steel Protection is active. You will have about 4.5K HP and 200ish energy.
Charge -> Perfect Protection -> Inspiration -> Spam Crushing blow, shield blow and anger. Once perfect protection wears off, spam charge again as well. Eventually you will need to use your HP and Energy potions, wait until the last moments before using them. Don't use Inspiration again until it is about to wear off and remember to use perfect protection as soon as it comes off timer.
A dirty little trick about the boss is that it will begin to quickly regenerate when it is down to about 10%-20%. At about 20% use Battle Shout, this will reduce it's regen by 50% for 5 seconds - which at this point you will be very low on HP but with a little bit of luck it should pop.