How to change the game's starting level?

Hi! Still working on a wad I'm making, but I'm having trouble changing a few things in the MAINCFG file.

I'm not sure how to change the level the game starts on when you choose to play in singleplayer; other wads I've seen have the game starting on the first level they have added in, but I'm not sure how to do it myself (scanning through said wad's MAINCFG files haven't turned up with anything.)

If this is a page on the SRB2 wiki, please link me to it, as the entire thing is a maze to me and I'm not sure how to get anywhere. If not though... help?

you're going to want this page: http://wiki.srb2.org/wiki/MainCfg

Thank you!