To create the calendar use a spreadsheet - I personally like Google Docs because the spreadsheet is there wherever I am and can be easily shared or viewed by others that you may be working with - but even an old school paper calendar will do. Set aside one hour at minimum to brainstorm and plan the content you aim to highlight in the upcoming days, weeks, or even months. Take into account any significant events that may be taking place over the period you decide to plan for, ie. holidays, special business events, local events, conferences, product releases, and other occurrences that might have a significant social buzz. By including all of the information and links to relevant sites in your calendar, you will have developed a single resource that you can refer to when deciding what to post. For Facebook you should be aiming to post at least once per week and probably not much more than 3 times per week, and for Twitter I try for at least one tweet per day.
If you're using a tool like Hootsuite to manage your social media you even have a calendaring tool built right in where you can schedule tweets or facebook posts ahead of time.