Learn German: Du wirst eine tolle Karriere machen.

This sentence uses the future tense (Futur I) to make a prediction. In German, you form the future tense by combining the helper verb werden (here conjugated as wirst for du) with the infinitive of the main verb (machen). Just like with modal verbs, the conjugated helper verb takes the second position in the sentence, and the infinitive gets kicked to the very end.

The English translation says "have a great career," but doesn't machen mean "to make" or "to do"?

Yes, machen usually means "to make" or "to do." However, German uses different verbs than English for certain everyday phrases. While in English you "have" a career, in German you literally "make" one: eine Karriere machen.

Why does the adjective toll have an -e at the end (tolle)?

Adjectives get special endings when they come directly before a noun. Here, Karriere is a feminine noun (die Karriere). Because it is the direct object of the sentence (accusative case) and follows the indefinite article eine, the adjective toll takes the feminine accusative ending, which is -e.

Could I just use the present tense to talk about the future here?