Can I upgrade to MacOS 10.14: Mojave- 2018.

My Mac book is an old mac book and I did get a new Hard drive and Ram put in and the computer is running a lot better now. But I would just like to upgrade it to the latest Mac OS system I can, but I can't Upgrade it to Sierra High or Mojave because I can't find a link to download. Any help? All the links I find just send me to apple's latest upgrade. Which won't let me download saying it's not compatible.