LogFAQs > #970128639

Yeah, it is really bad at explaining itself (though I will say that the first merchant does have a note you can buy that directs you to the phial).

As for the letter rankings, that tells you which stat goes with that weapon. The Estoc, for instance, has a C in Dexterity and an E in strength. So its damage will scale up decently when you increase your dex, poorly if you increase your strength, and not at all if you increase any other stat.

Other stuff that confused me when I started:
Ashes of War are reusable. Connected to the last points, Ashes of War can change the "affinity" of a weapon which changes the weapon scaling. For example, Storm Stomp is likely the first AoW you'll find. Aside from adding the Storm Stomp ability to the weapon, it'll also give it the Quality affinity which adds/balances Str/Dex scaling. So our Estoc goes from C in Dex and E in Strength to a D in both. Later on, you'll find items that let you more freely mix/match AoWs and affinities.

The damage numbers on the weapon show up as, say, (107+ 92). The first number is the base damage of the weapon that you can only increase by upgrading the weapon. The second number is how much the damage is increased by your stats.

Your weapon can deal multiple types of damage simultaneously. If you put a Fire affinity on an Estoc, you'll deal both physical and fire damage which resistances applied to both of these. That'll show on the weapon screen.

If you equip a weapon of the same type in each hand, you unlock a special moveset on the L1 button that utilizes both weapons at once. This is effectively how you dual wield in this game.

Spreading your stats out isn't too bad at the beginning while you're trying out different builds, but you absolutely want to pick a build and stick to those stats at some point. If you're leveling more than two damage stats (Str, Dex, Int, Fth, Arc), you're going to be much weaker than you otherwise would be. You'll unlock the ability to respec later in the game, so don't worry too much.

If you go strength, two-handing a weapon multiples your Strength's effect on that weapon by 1.5. So if a weapon has a Strength requirement of 15, you can effectively equip it at 10 Strength, but only if you two-hand it. This also applies to weapon scaling.