Any advice about first steps on Spacehey ?

Yes, all public blog posts appear in that section, and you can follow anyone's blog, doesn't have to be a friend.

That said, a forum is a forum. Never seen one before? It's kind of the same, with chronological posts, but conversation is the whole point, not a bonus like on blogs.

So yep, you've come to the right place.

As for groups, you can simply join any of them, no questions asked. Groups have their own forums and bulletins, but not much else.

You can join as many as you want, by the way.

And yeah, "kudos" are basically likes/faves, except you can give two of them to a post, not just one. That's the difference. They count double.

Hope this helps!

Blogs

Blogs are semi-public. You can change the visibility settings for specific posts so that only friends can see them, only someone with a direct link can see it, or even lock a post so that only you can see it.

Forum

The forums are completely public. Think of it like a giant comments section with threads that can go on indefinitely.

Groups

Groups are very comparable to subreddits, you are absolutely on the mark there. They each have their own sub-forum and bulletin board as well, and can be closed-access. From what I understand, there are a number of private groups that people create for them and their friends so they can interact without some rando popping in.

Kudos

It's like giving a like to something. For blog posts, it means the post gets pushed closer to the "Top Blog Entries" for whatever category it's under. I don't know why you can choose to do 1 or 2 kudos, that might just be a holdover from the site owner experimenting around early on.