To modify a file you will need to check it out. This can either be done by right clicking on a file and selecting 'Check Out for Edit' or letting Visual Studio automatically check out the file when you start to change it.
When a file is checked out it's icon in the Solution Explorer will change from a lock to a tick.
Checked out files will also be listed in the Pending Checkins tab as 'Content' changes and can be submitted to the server in the normal way.
The Pending Checkins tab can be opened by selecting the 'View | Pending Checkins' menu within Visual Studio. This allows you to keep track of which files you are changing.