Both the previous answers are correct, you need to have done the lvl 19 quest to attach materias(it's the same questline that lets you convert your gear into materias when it's spiritbound, it's in Central Thanalan, a bit northeast of Black Brush, Mutamix the goblin gives the questline) and then you need to either be a crafter or find a crafter of the same level and type than the item you're trying to meld(for example for relic, a lvl 50 crafter that can make the weapon, you can tell by the craft required to repair it which is written in the tooltip).
Finding a 50 crafter shouldn't be too hard but could be annoying now that most people left Mor Dhona for the time being. Just ask around in shout, most people will meld for free or ask for a small fee(1-2k, maybe 5k if you're desperate). You will also need Grade 3 Carbonized Matter, can buy those on the broker or ask the crafter as most of them will have some, it's like 200ish per but you might have to buy a stack which is annoying. You can Request Meld as an option when targetting a crafter, so you don't need to give them anything, it's automated, he only has to accept.