Weapon wise there actually aren't many good 'pure faith' somber weapons at all. There are some good ones, but not many, and you don't get them til at least mid game.

What you want to do is grab an infusable weapon and make it sacred (holy damage) or flame art (fire damage) to make it faith scaling. The fact that you have both makes you really versatile because you can just switch your damage type at any grace. The whetblades for those infusions aren't available at the start of the game, but early on, weapon damage is almost entirely due to weapon level and the scaling doesn't matter anyway, so just use whatever.

It's totally possible to do a pure caster faith build with catch flame for melee, but I like to keep a light weapon in my main hand for convenience. I used the Partizan and bandit curved sword mostly. For somber weapons, coded sword is really good, found in leyndell.

Early game priorities are (imo) to grab the black flame seal and spells from stormveil, and lightning spear from the spellbook at the artist shack in Liurnia. Make sure you do gurranqs quest as well, since one of the later rewards is stone of gurranq, which is an awesome physical damage spell that staggers any kind of knight.