Casio's Japanese domestic market watches come with a Japanese manual. If you can't read Japanese, go here:
https://world.casio.com/manual/wat/
Select the language you want, and on the next page enter the "module" (quartz movement) number engraved in a little box on the watch's case back. The module number of this watch is 5114 and you can download an English-language manual here:
https://support.casio.com/storage/en/manual/pdf/EN/009/qw5110.pdf
The sub-dial shows which function is active on your watch. For example, when using the stop watch function, the sub-dial hand will point left to ST. This watch has a lot of functions: stop watch, countdown timer, auto light, four alarms and a snooze alarm. You can manually set the time and date and adjust the position of the hands; and there is a quick way to switch between your home city and another. Check the manual and see if this is what you want.
This watch has standard lugs for a 20 mm strap or bracelet. 20 mm is the most common "man's" size of factory-made straps and bracelets, and every dealer has a big selection. If you need something extra long or extra short, the biggest selection is in 20 mm. Buy from a dealer who gives you two spring bars or will sell them to you cheap. I have never removed a steel bracelet without bending at least one spring bar.
You remove and install a standard strap or bracelet with a spring bar tool.
The big fork is for leather and rubber straps; the little fork is for steel bracelets and buckles. Bergeon makes the best watch tools, but you don't need a $20 Bergeon spring bar tool: a $2 Chinese tool works just as well for one watch. Some dealers give them away. If you buy a strap or bracelet from a jeweler or watch maker, they will install it for you and resize a bracelet.