It seems that sometimes the 'ji' ending is used for "I", but sometimes for "they". for example later on in this module the kids "maji".. can someone clarify?
You need to pay attention to the accents. :)
POTŘEBUJI VAJÍČKA = I NEED EGGS
POTŘEBUJÍ VAJÍČKA = THEY NEED EGGS
No, there is always a difference in vowel length - pay attention to the accent (čárka)! The third person plural ("they") ending is usually -í/-ejí/-ou depending on the verb class. On the other hand, first person singular ("I") ending is usually - i/-ím/-u depending on the verb class.
Both are accepted. Potřebuju is more colloquial and preferred in spoken language, potřebuji in written language.