Hiya! When I first started playing RO I was in the same exact position as yourself, everything was completely overwhelming and foreign to me. Even now, after putting a fair amount of time into learning all the different aspects of the game, I still ask A LOT of questions. When I first started out I just made a character that I thought looked cool and avoided characters that didn't. I started out with a swordsman, then made a priest for self buffs, then a sage for more buffs and then a merchant and it goes on and on. I avoided making a white smith cause I didn't like the way they looked but now almost a year later I main one almost exclusively. Point is, don't worry too much about what character you make because it's bound to change down the line. Read the wiki descriptions on these different classes and make your decision on what looks enjoyable and as time goes on you'll discover what class to make next depending on your current needs.
When it comes to leveling, there are a few posts in the wiki that should prove very helpful to you! You have the starting class player guide: https://wiki.talonro.com/Starting_Class_Player_Guide and the leveling guide: https://wiki.talonro.com/Leveling_Guide. For more class specific guides submitted by fellow players and staff, check out: https://wiki.talonro.com/Category:Class_Guides. There is a lot of great information tucked away on the wiki and to help relieve some of the immense confusion this game can bring I'd highly recommend giving it a look.
Weapons: Different classes can equip different weapons and those classes able to equip any specific weapon are listed on the weapon itself if you right click it. It will list a few other things as well like the weapons attack damage, weight, required level to equip it and the weapon level itself. The weapon level will be between 1-4 with level 4 being the highest. I'm not exactly the most knowledgeable on weapons so a basic explanation of it is: level 1/2 ain't worth yo time(in most situations). Levels 3 and 4 weapons are typically(can't speak for all classes) the one's you'll invest in to take with you to mid/late game. The level of the weapon also influences the type of resources needed to refine the weapon with the NPC in prontera or by a whitesmith. Each refine adds additional bonus attack damage to your overall ATK stat(will show up as +### on your stat page) with level 3 weapons adding +5 per refine and +7 for level 4. Must also be noted that you can also refine level 3 weapons to an overall higher refine without the risk of breaking the weapon for going over the safe limit but the max refine for any item is +10 regardless of level.
At some point in the future you'll start looking into adding cards or enchants to your weapons/armor depending on what you plan on using it for. Enchants are something you can worry about once you get your bearings for TRO, but cards are something you'll need to familiarize yourself with early on. You may run into an item with a slot in it which is identified with a number between 1-4 in brackets. An example of this would be: Hood or Orcish Axe. With the hood, you could socket a card into it as long as the card you wish to socket says "Compound on: Garment" in the description because the hood is equipped in the robe/garment spot. The same applies for the weapon except you can have 4 unique/duplicate cards in the Orcish Axe. To actually socket a card you need to have the piece of gear you wish to socket unequipped and in your inventory and double click the card you wish to socket. That will bring up an options menu for you to select the piece of gear you wish to compound it to. Once you socket a card though, it's in there for good unless you pay Talon Coins(this servers voting/donation reward currency) to have the card taken out of the gear but the cost is 5TC. So choose wisely if it's worth replacing it!
As far as prices go on the market, I couldn't tell ya! People post things for wonky prices all the time and undercut each other by disgusting amounts. The best way to understand rough price estimates for different items is to just familiarize yourself with their uses in game, is it a popular item, how many talon coins does it cost to buy it from the rewards guru and their demand. Could also lurk the market place on the forums to try and get an idea for specific items. Or ask people questions! I bombard Wormi/Lacrimosella with questions about prices all the time and it's a testament to her patience that she hasn't blocked me yet.
I hope I gave you a decent answer to some of your questions and I'm answering all of this off the top of my head so I would verify the different numbers/processes/uses I gave you through the wiki! It really does hold a massive wealth of information as do the different class forums, though it may be a bit hidden. If you have any questions you can hit me up in game on Watlende or Takanti.