>What does each element do?
nioh.wiki.fextralife.com
>What should I do with the armor/weapons I don't want/use?
Check if you can donate any at the hidden teahouse for daily/clan rewards. Otherwise preferably disassemble them. You can also sell them to Tome or donate to Danjo at the Teahouse if you need a bit of extra glory or cash. Later on you'll be keeping them for Soul Matching, but don't worry about that until you get to NG+.
>The blacksmith is expensive, should I be using it?
You don't need to. Throughout WotSamurai and WotStrong you will constantly be finding better gear and builds don't matter so much. Soul Matching is expensive and intended for late game. If you do find a weapon you really like, try reforging it's effects using spirit iron, it's the cheapest and easiest way to improve a weapon.
>Why do people keep mentioning "smithing texts"?
Bosses tend to drop pieces of their armor and weapons when you defeat them. Very rarely they will drop a smithing text which allows you to forge their armor/weapon.
>Which clan should I join? (clan unlocked through progression)
Whichever one has a bonus that you think will benefit your build the most.
Beginner's guide:
pastebin.com
FAQ:
pastebin.com
Smithing text list:
docs.google.com
Farming smithing texts: pastebin.com
Unique helmet list:
imgur.com