I will briefly mention, What has been said, before. Studying is truly the best option, but it is also the hardest. Your mind won't always focus, which is why I think your asking for herbs.
I say try peppermint tea, with a little bit of ground cinnamon. The peppermint is too help with focus, and psychic ability. Along with the cinnamon, it will produce calmness though the body.
Also try success oil, rub it, into a green candle. Whenever you study, burn it. Say a chant too, like this one. Credit* Green wiccan book of shadows(study focus)*
"I need to focus, it is a must, so dont let this study, be a bust, thou it's hard, i wont restrain, i will memorize, and remember without pain, mote it be!"
A few other herbs can include:
*Bayberry(prosperity, and focus)
*Lavander (calm peace, rememberance)
I suggest trying to spray some lavander scent around your room, light a candle thats green, or yellow(prosperity, wisdom)
And just try to study.
Another way is to wear good jewelry, or metals when studying. I suggest the following clothes, and metals.
Tin necklace or bracelet
Lead(could be dangerous on skin, so hold it in a package )
Some stones include:
Quartz(focus, rememberance, competition)
Cloves (the plant I mean)
Obsidian(good for grounding, focus, calmness, and remembrance )