I already created a website in Dreamweaver but it is not centered on large screens and mobile screens.
Can you tell me what I can do to make that work better? Thank you.
You need to make yourself familiar with CSS Media Queries (or the Fluid Grid Layout system).
It's most likely you will need to redesign your site to make use of them though. It's pretty rare to build a static desktop site that can easily be converted to an MQ based site.
Here's some good info on how to work with them in DW:
Or look at the F1 Help from DW for Fluid Grid Layouts (I'd say if you have some code experience, write your own).
You need 3 things to center your web page.
NOTE: This does not make your site responsive on mobile and tablet devices. For that you need media queries.