There's only one mercenary at any time, and you'll want to select one that is compatible with your character. Consider their skills when you're looking to recruit one; a rogue with Cold Arrow (which slows enemies but does less damage) D2R Items could be ineffective for you if you're a Sorceress that is already slowing enemies or a necromancer with summons that divert enemies' attention.
Once you have a mercenary, make sure to outfit them with equipment to increase their effectiveness. Also of note is that ethereal objects, which can't be repaired and are not repairable, can be useful for mercenaries due to the fact that the items that are equipped with mercs don't degrade in durability.
In the beginning you'll start to collect gems that can be put into items with sockets. They are extremely useful when it comes to socketed weapons, which in particular can offer a substantial damage boost. Even if the basic characteristics of an item aren't the best take note that the presence of a significant amount of sockets could be transformative when used with care.
This is the place Runewords are available. As with the other gems you'll find, runes can be placed into sockets to provide positive effects. However, if particular runes have been inserted into buy diablo 2 resurrected items items in a certain sequence, you'll be able to activate a Runeword.