Whenever I go to a channel for soemthing they always have a notice-y type of thing that everybody can see when they first come in..How do I do that?

Hi!

An "on join" notice? Well, it could be annoying for some people, specially the channel ops, it could be considered as spam if you include a web link.
Here's the code you have to put in the "remote" section (alt + R)

on *:JOIN:#:/notice $chan Welcome to $nick !

This notice will be sent to the whole channel, it's quite annoying if a lot of people is joining a channel.

on *:JOIN:#:/notice $nick Hi $nick and welcome to $chan !

This one will be sent to the joining user only.

by the way, everything you need is in the mIRC's help file, just type: /help on join

Just an added note, make sure that if you do add this to your remotes, that you only do it if you're an op in the channel or have permission from the ops of the channel. It is general 'netiquette' that you do not have auto-join messages enabled in other people's channels.

A way to make sure you don't forget would be a tiny modificaiton to the above code:

on @*:join:#channel:/notice $nick Hi $nick and welcome to $chan $+ !

The @ means it will only activate if you're opped in the channel. The #channel means it will only trigger on the channel you specify. So, if you put #demochan you would only send an on join message to people that join #demochan, and only if you were opped. The $+ just makes the '!' sit next to the #channel name instead of having a space.

Have fun!

Regards,

just as an added note too. A "join notice" may be set up from the IRC services as well (depending on what type and version it is) try typing /msg chanserv help and see what happens