I have a old mac book pro computer that has a safari 12.1 that i need to have a 14 plus safari. when i go to apple store for app it saids it won't work on my computer. Do need to buy a new computer.

The model year of a MacBook Pro is available at Apple menu > About This Mac.

If your MacBook Pro cannot be upgraded to Catalina or later, your cannot get the current version of Safari. See: Apple security updates - Apple Support

If you cannot upgrade, one option is to use a 3rd party browser that allows a current version to be installed. The current version of Chrome, for example, will run on OS X 10.11 or later.

Following is a list of possible upgrades for MacBook Pros:

An Early 2008 through mid 2009 MacBook Pro can be upgraded to El Capitan 10.11.6  but no later version. See: How to get old versions of macOS - Apple Support

A mid 2010 through late 2011 MacBook Pro can be upgraded to High Sierra 10.13.6 but no later version. See: How to get old versions of macOS - Apple Support

A mid 2012 through early 2013 MacBook Pro can be upgraded to Catalina 10.15.7 but no later. See: How to get old versions of macOS - Apple Support

A late 2013 and mid 2014 can be upgraded to the latest  version of Big Sur macOS 11 but no later. See:How to get old versions of macOS - Apple Support

An early 2015 or later can be upgraded to the current version of Monterey macOS 12. See: How to upgrade to macOS Monterey - Apple Support

MacOS also supports FireFox and other third-party browsers. Probably a new enough version for your needs will run with your current MacOS.