Hello Carlos the Jackal,

First of all you need at least a green quality weapon or armor. If you click the upgrade button on one of equipment pieces it will open up a new window. In this window you can check what 'slots' item you want to upgrade has. Each mark mentions what slot it fits. Mark should be equal or lower level than the item you are trying to fit it as well as being for same 'class'. Marks that you can fit will show up in this improvement window and you will be able to 'fit' them! I hope this helps! If you have more questions you are more than welcome to ask here or message me in-game!

Still a little unclear..... will marks make become available to me throughout the game? Or is there a specific way to obtain such "marks"?

Green marks are dropped of usual monsters and elites. Blue marks - from elites , boss monsters in instances and in future from event bosses and Purple marks available once you have enough reputation. Marks can be equipped to transferable items and you will still be able to trade these items. However if the player doesn't have enough reputation for mark equipped on item - it will simply not give bonus to that person. Marks can be removed for small cost , but upon removing they lose their power and disappear. There are also many marks already available on Auction House sold by players.

Other info:
Maximum protection from any magic school - 75%
Maximum reflection from any magic school - 30%

Values higher than those mentioned above will simply be cut.

Mark should be equal level or lower than item you're trying to equip it.
Mark should be same class as item piece you want to equip it to.

that cleared it up quite a bit, thank you for your time