Hi, .NET Foundation Executive Director here, happy to answer any questions. Also, check out Miguel de Icaza's post here for details and background: https://tirania.org/blog/archive/2018/Dec-04.html
1. Start with conversation - it's much better to start with a new issue or a comment on an existing issue, letting the repo owners know that you're interested and want to contribute to a specific issue. They may give you tips, let you know that the issue's already being worked, etc.
2. Docs are a great way to get started. You can go to any of the .NET Core / ASP.NET Core docs on docs.microsoft.com (e.g. https://docs.microsoft.com/en-us/aspnet/core/), click the edit button, make your corrections or contributions in the browser, and submit.
3. Many projects use the up-for-grabs label for issues that are easy for first time contributors, e.g. https://github.com/aspnet/AspNetCore/labels/up-for-grabs (again, start with a comment on these, since some of them are a little old and may no longer be relevant)
4. Don't just think about the big name .NET projects. Membership is open to contributors to any .NET Foundation project, including a lot of projects with small teams that would love some help. Check out the full list here (https://dotnetfoundation.org/projects), or tweet me with hashtag #dotnethelpwanted and I'll rt to help get you matched up.
5. Check out some of the help wanted listings, e.g. https://discoverdot.net/issues/?tab=helpwanted (remember we're looking for .NET Foundation projects for membership)
Even for bugfixes, I've been really dissapointed in their responses - they call any change in behaviour a 'regression', and won't accept fixes, even if the behaviour they want to preserve only exists as a side effect of the bug, and nobody could possibly want in any case!
I write this as a .NET fanboi, but my negative experience of trying to contribute to Microsoft's OSS projects has been left me dissolutioned.
.NET Foundation is an independent organization, with Microsoft as an important member and big contributor, but .NET is owned by the community. I've heard a lot of great ideas for how the .NET Foundation could support projects better, do more for our Meetups (including providing content, support speakers with things like speaker grants, etc.), put on in person events, etc. - but we haven't had a good way for people to make it happen, and the foundation could only do so much. Now, by letting the community guide the foundation through community board seats and bringing on corporate sponsors, we really can scale up so that if someone has a great idea, they can just do it!