"I drink milk so that I will be strong."
Translation:Saya minum susu agar saya kuat.
"Agar" should be used at the start of a clause. It can't be used after "saya".
Replacing "agar" with "jadi" would change the meaning of the sentence. "Agar" indicates a purpose and can be translated into "so that". "Jadi" indicates a consequence and can be translated into "so".
Although the English uses the word "will", it is unnecessary to translate it into "akan" in Indonesian. Remember that Indonesian doesn't have tenses, so the future tense in English doesn't always have to be translated in the Indonesian sentence.