I'd suggest searching the Internet, to be hones. Something that works well enough for most plants and herbs, I've found, is "magical properties of [plant name] ."
As for seasonal altars like Beltane, see what qualities are represented by that holiday, and choose flowers which are relatable. A quick search shows me things like fertility, spring, outdoors and nature. The wheel of the year is not a part of my practice, however, so I am not so intimately familiar with the holidays.