"A first post" -- makes sense or not?

Solution 1:

I agree with your first-poster. Both are possible.

I made a first post on my blog today.

Other people make first posts on their respective blogs too.

I made the first post on my blog today.

This one is fine as well; now the implicit context is only my blog.

I made my first post on my blog today.

This is possible, since it is my first post on this blog; but it is a bit awkward, because you have my twice. You could use My First Post as a title, though.

Solution 2:

Sounds fine to me, for the reasons given.

We have a first step, a first word, a first kiss, etc.