MyBB Community Forums

hi i am in first using of mybb
and i want to know how can i install plugins in mybb

thanks

Read our docs:

http://docs.mybb.com/Plugins.html

Most plugins come with a readme file instructing you where to install the files. Once the files are installed you visit the "Plugins" section of your AdminCP and activate them.

Read read read and practice on locahost via XAMPP for whatever OS you have.
If you are new to MyBB and are messing with an established forum it's best your practice on an offline forum first. If this is a new forum then by all means have fun experimenting, just upload the plugin files to their proper locations using your servers FTP or SFTP login credentials.
Like Jovan said, read the MyBB documentation.