lol, must have been half asleep when I read this. Yeah, milosh is correct. You could do an:
On *:TEXT:!topic*:#mychannel:/topic # Welcome to my channel. Enjoy Ur stay. News : $1-
Hope that completes any questions Milosh may have left you with. Good luck!
P.S. If you need to make it respond to you as well, this script must be run from another user OR add another line identical to that one that is On *:INPUT