I typed "Du beginnen". Beginnen was the suggested word. But I was told I'm incorrect and that it shoud be "du startst"
The suggestions are not answers - at best, they are tips that you also need to adapt to the grammar of the sentence. At worst, they are automatically-generated from a database and can sometimes be completely wrong. Just take it all as learning experience - mistakes are inevitable and unavoidable.
You need to conjugate the verb to match the subject. The subject is the thing or person doing the action. Conjugation is using the right form of the verb. Consider "I read" but "he reads". So maybe a dictionary hint for someone learning English would be "read", but 'he read' would be incorrect here. The matching form for du is beginnst. I recommend bookmarking www.verbix.com for when you need to look up a conjugation, but eventually you'll get a feeling for the patterns and exceptions.
As for why Duolingo suggested a completely different verb... well, when you give an incorrect answer it just gives you a correct one at random from its database that contains multiple possibilities. For more nuanced feedback you may benefit from in-person lessons :)
I wrote "Ihr beggint" and it came up wrong... but i feel like its right??
I wrote du beginnst but came to check if yours was also exceptable, and if not, an explanation why not.