You will need a Hedera mainnet account to interact with and pay for any of the network services (cryptocurrency, file service, and smart contracts). Your Hedera account is what holds a balance of hbar to be used for transfers to other accounts or payments for network services.
One of the primary means of acquiring hbar and a mainnet account for use on the network will be through an exchange. Exchanges typically allow users to create an account on the Hedera mainnet, and transfer in an initial amount of hbar in exchange for some other coin or currency. Some of those exchanges supporting hbar can be found here.
Users can also create accounts on the mainnet by visiting the Hedera Portal and creating a new profile. Please note that you will be required to complete a KYC process in order to do this and will be credited 5 hbars for your trouble.
Once you have joined mainnet, your account ID will appear on your home page.
Your mainnet account ID will be at the top of page in the purple box labeled “Account ID”. Make sure the mainnet drop-down is selected if your profile is associated with any testnet accounts.
Please note that accounts on Hedera (whether on the mainnet or testnet) are managed using key pairs. In order for the Hedera Wallet and most applications to successfully interact with the network a user needs to have the correct account ID matched with the correct key pair.