You might have the defult program to open java files as notepad, if this is the case then go to properties and select a new defult program, if it's already Java then don't mess with it.

Go here, https://optifine.net/home to download optifine, usally if you download it anywhere else it will probably crash.

Hope the helps.

the optifine download is an executable java file, right click on it select open with and select java, it will then start the install of optifine

if there is no option you do not have java installed and will need to install it