Since it seems you're extremely new to BTC payments (not even understanding the divisions of BTC until now), you should go for something user-friendly but still relatively secure since you're handling high amounts.

Try Electrum (and make sure you keep the seed safe - without it you can't access your coins).  A hardware wallet like TREZOR might also be a good idea for you and it should guide you through the process quite well.

Of course when selling items on the forum, people will be wary of sending their BTC to a newbie, so you'll have to make sure to use a trusted escrow or a multisignature system.