How much memory does your MBPro have? I've seen some of the regulars on here mention macOS 10.14 Mojave really needs a minimum of 8GB of memory.
AFAIK you should be able to upgrade directly to macOS 10.14 Mojave if you wish. I know there are a few systems where it is necessary to first upgrade to OSX 10.11 El Capitan. AFAIK unless you have already "purchased" OSX 10.10 Yosemite, then you won't be able to download it now. Here is a list of the remaining versions of macOS which you should be able to download:
https://support.apple.com/en-us/HT201372
The above link contains links to information for each version of macOS still available to be downloaded. Click on the macOS name to get a page containing the direct download links to the App Store. Sometime in October macOS 10.15 Catalina should be available to download and install, but Catalina will only support 64bit apps.
Before you upgrade your OS, make sure to have good verified working backups just in case something goes wrong. I also think it is a good idea to create a bootable macOS USB installer just in case something goes wrong.