There are many ways to make changes to the today screen. It's kind of a learning process. Most changes are made by various different programs...lots of programs are known as TODAY SCREEN ENHANCEMENTS like you can find
here. Many other programs have capabilities that can be displayed on the TODAY screen by going under SETTINGS, click on TODAY, click on ITEMS tab at the bottom, and turning them on and off. Some will open up the OPTIONS button on the right which will let you customize what is shown. There's also themes that can be d/l and applied and many other things.
What kind of device are you running? Do you have HTC Home on there?
If you do I would recommend downloading
HTC Home Customizer v1.0 as it's one of the easiest ways to make changes to that specific program.
Hope this gets you started...