Actions on a page can be called in a few different ways. You can call an action in your controller from the loading of the page via a page action or within the constructor when the page loads (it's worth noting that DMLs are not allowed in the constructor).
Lastly, you can use an actionFunction, which is essentially a Visualforce AJAX layer.
Here is a good help article explaining the visualforce order of execution on a page load:
Here is a link to actionFunction:
Because your question isn't quite clear, I'll offer one last comment. If you're just trying to interpret a field on your record, based on data from other fields or even the user, then a formula field may be appropriate. Formula fields are just custom logic to interpret the underlying data. They're not actually changing anything, they are just a static interpretation of data.