You can do this in quite a few ways.
1) mIRC provides an internal function for this. Click on the third button on the toolbar from the left. It's called the Channel Folder. Select or Add a channel of your choice and then click on "Edit". There's a checkbox at the bottom called "Join channel on connect".
2) You can use mIRC's Perform on Connection option to do this. Press Alt + O (or go into the FILE menu and chose OPTIONS). Under the CONNECT tree, there's a menu called PERFORM. Click on it. Check the box called "Enable Perform on Connect" and enter "/join #whateverchannelyouwant" into the big editbox.
3) You can add a line into remotes. To reach remotes, press Alt + R (or go into the TOOLS menu and pick REMOTE). Enter the following: On *:CONNECT: join #whateverchannelnamehere
. Click on OK to close the Remote window. Type /remote on
in the command line of the Status window or any other window.
You're all set.