If you have a Time Capsule on your network, you can use it as your Time Machine backup disk.

Once your Time Capsule has been configured, open Time Machine preferences and click Select Backup Disk. 

From the sheet that appears, select the Time Capsule you would like to use for backup.

Tip: Click "Set up Time Capsule" to open your AirPort utility to setup and configure your Time Capsule. Enter the name and password or password only that you set for your Time Capsule via the Airport Utility.

Selecting items to exclude from the back up

In Time Machine preferences (in System Preferences) you can click the Options button to adjust settings. A sheet similar to this appears when you click Options:

This sheet allows you to exclude files, folders, or entire volumes from being backed up. You might want to do this to avoid filling up your backup disk.

Tip: If you regularly modify a very large file (greater than 1 GB, for example), you might want to add the file to the "Do not back up" list. Time Machine backs up modified files, regardless of how much or how little the file changed from the previous backup.

The "Warn when old backups are deleted" option tells Time Machine warn you when older backups are removed from your backup disk to make space for more recent backups. 

Manually begin a Time Machine backup

From the Time Machine menu, choose Back Up Now, or right-click or Control-click the Time Machine Dock icon and select "Back Up Now."