Table Issues

I'm having some issues with tables. And cannot find an answer through the forum's search function.

(Bear in mind I'm extremely new to the scene and know little about creating webpages)

Im pasting tables I create in excel into a webpage. Everything looks great in the excel output. But when I load my page, the column headers move onto two lines instead of one and I can't figure out why, or how to fix it. (here is what I'm talking about)

Furthermore, is there a way to make it that so when the page isnt maximized, the table doesnt get cut off by the sidebar?

Thank you