Crafting and gathering are still very much optional content in the game. You can level adventuring classes up to the highest level and complete the hardest fights without ever even opening the crafting or gathering windows(i was going to say without even visiting their guilds, but a couple of them are visited during the storyline on various fetch quests). You could choose to level up just gathering, and sell your wares on the MB for profit. Most people who take up gathering do so to make crafting easier by getting some of their own materials, but profit is certainly a viable motive.

As for materia, it isnt "crafted" exactly. Some equipment can be converted to materia after it gets to 100% spiritbond. If you look at the item description, there's an entry that says whether or not the item is "Convertible" - thats how you know if it can be changed. Theres a quest in central thanalan at about level 19 that gives you the ability to do these materia conversions. If you were asking if other people can put materia into your gear for you(or whether you have to do this for yourself), then the answer to that is yes. Other players as well as some NPCs in town can attach materia for you if you dont want to level up crafters to do this for yourself. I would also point out, materia isnt really needed right now for anything short of some end-game tasks. So it isnt something to worry about for awhile yet.

Or should I put crafting/gathering on the back burner and focus on leveling the main job first?
Thats something only you can decide. You do have to complete the ARR - patch 2.55 storyline to get into ishgard and the HW areas, so at some point your gathering leveling will be stymied because you wouldnt be able to get to the new areas. But really, aside from access a high level adventuring class only offers small benefits to leveling up a gatherer. So id say do whatever is most fun.