To know what a noun is for a normal three letter verb, the best thing to do is either search a dictionary or ask an Arab.
This is because there are no strick rules for it so it's just something you need to know.
So the noun for أرسم is رَسْم
The noun for أدرس is دَرْس or دِرَاسَة
The noun for أشرب is شُرْب or شَرْب
The noun for أرجو is رَجَاء or رَجَاءَة or مَرْجَاءَة
Sometimes there's more than one.
These are called "Verbal Nouns".
Nouns which come from verbs.
So yeah, just ask or search in a dictionary. A nice dictionary is Almaany.
"Studying Arabic is hard" you can say
دَرْسُ العَرَبِيَّةِ صَعْبٌ