In order to unlock melding, you have to be level 19 in one of the gear-making classes (i.e. any crafting class other than Culinarian) and take the "Waking the Spirit" quest. But beyond that, there are class and level requirements for each piece of gear. You'd need the appropriate crafting class (the same class that makes the equipment) leveled up to the level of that equipment in order to meld materia onto it. Either that, or get someone who does have the appropriate craft/level to do so for you.

(If getting someone to do it for you, the "Request Meld" feature lets them meld an item of your choice from your inventory with materia that's also from your inventory. You don't have to pass the items to them. This both prevents unscrupulous players from stealing what they offered to meld, and solves the issue of getting bound items which you cannot trade melded.)