Assumes you have Windows 8 or later, which greatly improved the built-in accessibility features.
Turn on Windows Magnifier. Set it to 100% - that is, no magnification. But in Settings, check on "Turn on colour inversion". Your screen is now mainly white-on-black and less glaring.
Option: Instead, select one of the High Contrast themes. Not all applications will respect this, however (Chrome offers you a High Contrast extension, for example).
TEXT SIZE - GLOBAL
Reduce your screen resolution (not the text scaling, the actual number of pixels horizontally and vertically). This works fine in every application, where changing the text scaling doesn't work across every application. It's the simple fix I employ most often.
Option: also check out the Display > Change the text size only options, which give you bigger title bars and suchlike.
TEXT SIZE - PER APPLICATION
Yeah, this is the tricky one. You don't want to have to scroll left and right: as you say, "make things fit inside the screen".
Microsoft Office - zoom controls, bottom right of each application, combined with "Draft" or "Read" modes in recent versions to give you more text and less pretty whitespace on the screen.
Browsers - check for "Readability" functions, either built in or as extensions: page loads, you click on them, text all fills the screen, flowing and readable and sizable without left-right scrolling. "Reader View" is the one in Firefox, icon in the address box. "Reading View" in Edge, same effect. The Readability extension in Chrome.
Option: try using the mobile versions of websites, like mobile.facebook.com, which have simpler layouts assuming less horizontal space and therefore zoom better.
PDF - Adobe Reader - F4 to make PDFs reflow, normal zoom controls then zoom text without left/right scrolling.
Your vision may degrade further as you age. Check out NVDA (open-source free screenreader) and WindowEyes (commercial but available for free for anyone with a copy of Microsoft Office).
I work in assistive technology and have developed the open-source WebbIE software for fourteen years for blind screenreader users: http://www.webbie.org.uk/