Magic is an entirely optional part of the game, but it is very useful, and even melee focused characters can get a lot of return out of a few points put towards Int, Faith, or both.
Unless you’re intentionally challenging yourself with a specific character concept, it’s a good idea to make use of as many spells as you are able—while straight offensive magic tends to be less useful without a lot of points invested, supportive and healing magic is extremely helpful for any character.
Just because you’re a brute with 40 Strength doesn’t mean you can’t heal up between fights with a bit of magic!
Using spells is as simple as equipping a proper Catalyst and attuning spells at a Bonfire. Staffs are used for Sorcery, the Pyromancy Flame for Pyromancy, and a Talisman for Miracles.
You can equip a Catalyst in one of your secondary slots and swap to it as needed. If you are playing an actual combat caster, you probably want to keep it in your main hand so you can still use a shield. If you’re mostly using out of combat support magic, it’s usually safe to swap out your shield or one-hand your giant weapon for a moment.
You do add on a bit of extra weight to use a Catalyst, but it’s a small price to pay for the benefits. Much like ranged combat, spells are another facet of your character’s power that you would do well to take full advantage of!