how to make a custom message with doom builder 2?

The intermission story screens are separate from maps, you you can't change them with Doom Builder. If you want it to be vanilla-compatible, you can make a DeHackEd patch (WhackEd is the Windows utility for that) to replace the text (open the "strings" section, and then scroll down to the story text strings near the bottom), and then have the players load the resulting .deh file along with your .wad.

im in whack ed right now editing it!

uh i got a problem i know this is basic but i can't access my wad or really any doom files since they don't appear on whacked4 any idea how to fix this?

Just use MAPINFO to do it with clusters enter/exit text property. A lot easier than using WhackEd.

and how do i do that?

Open zdoom wiki, it has everything what you need related to mapinfo. You can use slade for that lump. 

As others have said already, for vanilla or boom compatibility you're gonna need a DeHackEd, for anything ZDoom you're gonna wanna use MapInfo.

Bear in mind that making choices with regards to which mapping format you use also impacts what you can/need to do how when it comes to editing things, be it the map itself or anything else like menus or intermissions. Personally I feel like editing intermissions is something you should concern yourself with after some smaller releases to hone your mapping skills by way of racking up feedback, but if you're that far into your WAD, might as well do it while you're at it.

As a rule of thumb if you're a windows user like me: SLADE is what you're gonna have to use for everything related to mapinfo, changing music, importing and or editing custom textures and so forth. If you want to, you can also use SLADE as a map editor if you like the interface, however most people prefer either Doombuilder2 (or its latest fork made by forum member anotak), or GZDoombuilder for the purpose of editing maps.

If you really wanna get "deep" into mapping, you'll eventually try and experiment with different mapping formats and editors anyway, so don't sweat it.

everyone keeps telling me about map info the thing is i have no idea how to edit map info.

You have already been advised to take a look at the ZDoom-wiki to learn about this, which imo is the best advice you can hope for, since you're mapping in ZDoom format IIRC. What more do you expect?

MAPINFO is just a text lump. Most maps for G/ZDoom make use of it.

Of importance for your purpose are Map definition and  Cluster definition.

Maybe this will give you a leg up on the subject.