That is a good Q.
Outlook desktop have own repository to code VBA, If you write there something you can send this file to someone ho do not use it at all.
That is inconvenient but it works. I start like that. I build Ribbon and tools i use at work. And I send it to my coworkers.
After I new how the code works and where is limitation, I bought Visual Studio to make COM add-ins.
VB.Net is a very similar but you can find lots of deference. Another way to build menu, set to object are deferent and you had a more it for use.
Long way to success, but i like it.
Now I have over 40 of it works and trade as a outside product for me :P
So if you're asking how start - so that is my way.
Regards