For PC boards you want a 30 watt "pencil" iron. Ungar, Hexacon and Weller are the quality brands, but Radio Shack is OK. Key item is that you can buy replacement tips, which you can't do for the $6 Chinese brands. Ideally you want a variable heat unit with a temperatue control, but those cost money. I got mine at a tag sale.
30 watts is enough to let you solder small wires like HU harnesses too, but not enough heat for the big stuff. The mainstay in my soldering arsenal is a Weller 100/140W gun that I've had for probably 40 years:
Weller Universal Multi-Purpose heavy Duty Soldering Gun Kit 8200PK | eBay
On the hi heat it'll solder 8 gage wire if you know what you're doing.