Which scenarios (or even campaigns) would you recommend for a beginner?

[Wdll]

First of all, IMO, you have to choose time period. Vietnam War era and Modern, are not the same in how you do almost anything.

I can't recommend a scenario because I have only played like 2-3 of them in all these years.
If say you want to play in 2010, then I would suggest South Korea (you) vs North Korea. Or Greece vs Turkey, or UK vs Iran.

Then it is the matter of scale. For 2010 games I prefer 30000 points + and max map, but you might want to start with something like 10000 points and smaller map.