Help how?
A mod launcher would basically be an application that looks for your SRB2 folder, lists all available mod files, and then lets you select which ones to load and a button to launch the game, or at least that's what I imagine a basic launcher like that would be like.
To make such an application you would need knowledge in any programming language that has some sort of library to create user interfaces, or just about any tool for creating them, and then knowledge in such language to iterate through files and folders, parsing what's useful and what's not, and the such.
What you're asking is too broad to give you any specific help at all but "look for it yourself".
You could try looking for wxWidget for Python, C# for... uh... or wxLua for Lua.