Mohsen_Nirouzad
If you have no idea what composer is you probably should not install Flarum now.
Anyway maybe it's a good idea to check Composer website first (https://getcomposer.org/) so you can understand what it really is.
Then check with your host if you can access your server via SSH. if you can, you should log in to your server via SSH to install composer.
Or you can simply install composer in your local computer then install Flarum with composer. then upload all the files to your online server. I don't know if it's possible but it should. I haven't check yet..