Need solution for Mac Mail backup..!!

Export your mailboxes. You can import them later if you want to access the messages

https://www.cnet.com/how-to/a-better-way-to-archive-email-in-apple-mail/

Archiving old mailboxes is an interesting idea, but you should first look at other features provided inside Mail.

The way Apple expected you to deal with this is to create file folders (new Mailboxes) inside the mail Application. These show up under the heading "On My Mac". When you copy emails from your inbox to these file folders, they will eventually (depending on your settings) be removed from the inbox on the email Server, freeing up space.

These additional "On My Mac" mailboxes can be searched with the standard mail search, so that you can find your old correspondence quickly and easily, and use it "live" (such as creating a new email) without resorting to a restore from Archive.

Being New to mac is a mess, i can understand that. But its not that hard to learn all the stuff, I see a couple of best solutions are provided by Illaass and Stenash. I can only recommend you to search your way through google and judge for yourself what is the best: Apple Mail Backup