For anyone stepping into the world of 'Rosario + Vampire', the sheer number of manga volumes and the sequel series can be downright confusing. I spent ages piecing it together myself. The absolute core is the original 'Rosario + Vampire' manga, which is two seasons long – that's volumes 1 through 10. That's your main story about Tsukune at Yokai Academy. After that, you absolutely must continue with 'Rosario + Vampire Season II'. It's not a separate story; it picks up right where the first season ends and runs from volume 11 to volume 24. This is where the plot gets seriously intense, with the Alucard arc and major power-ups.
A lot of people get tripped up because they watch the anime first. The anime is fun, but it becomes a completely different, filler-heavy story after the first few episodes. It totally diverges from the manga's plot, especially in its second season. So, if you're coming from the anime, treat the manga as a fresh start. Read from volume 1, even if you think you know the story. The tone, character development, and actual plot progression are worlds apart. The manga gets surprisingly dark and strategic, which the anime barely hints at.
Stick to that order: the 10 volumes of the original series, then immediately into the 14 volumes of Season II. There are also some side stories and anthology comics, but those are purely bonus material for after you're done. They won't make sense or have the same impact if you read them out of sequence. The main through-line is so tightly written that jumping around would ruin a lot of the built-up tension and character revelations.