Your question has the tag WebDriver but in this case I would consider the Selenium IDE
Download and install the Firefox Selenium IDE plug-in - Selenium IDE
Start using it and learn how to use its functions to:
- Visit a page
- Click on elements
- Enter data in input fields
- Select dropdown options
- Submit forms
A final note - I love browser automation but I wouldn't use it for time sheet filling.
Too likely to have a unnoticed error IMHO. I'm sure it would start at 8 hours every day, never changes but at some point you or others would hit a snag, incorrect or invalid data and when it comes to timesheet entry that can make a real mess and get you into trouble. Your explanation that you were using automation to fill it out would just make the situation worse.