you can choose from many options depending on the country that you live in or your preference.
you can use a service like coinbase or circle just make sure to read their terms to see if it suits your usage, also you can use from many exchange services like kraken, bitfinex, btc-e, ... to use credit card or bank transfer,... or you can use services like c-cex.com to make deposit/withdrawals to online payment processors like okpay,...
or you can find someone close to you in localbitcoins.com and sell them with cash.