I'm a new comer to Apache. I wish to redirect all of the links from the domain to particular file domain.com/welcome.html before visit every other page.

Think about using mod_rewrite:

http://httpd.apache.org/docs/current/mod/mod_rewrite.html

the proper mechanism to redirect incoming traffic to particular page with different number of conditions (incl referrer url, user agent, etc).

.htaccess is all about authorization of assets, not redirection between assets.