To start bidding and purchasing art, you'll first need some Ether (ETH).
Here are two exchanges where you can convert currency like USD, EUR, YEN, etc to ETH:
Move ETH Funds to Wallet
After you've created an account and funded it, you'll then need to transfer those funds off the exchange to your crypto wallet.
Actions like bidding, buying, or transferring NFTs are examples of transactions.
See also: How do I create a crypto wallet?