This is a word of masculine grammatical gender, therefore the correct article is ο. The word itself ends in -ας, nothing you can do about it! Feminine word examples: η γυναίκα (the woman), η πόρτα (the door). Neuter word examples: το παιδί (the child), το κορίτσι (the girl), το αγόρι (the boy), το τραπέζι (the table). Be patient, the more lessons you see the more gendered articles (and word endings hopefully) will start making sense. Happy learning!
Άνδρας is the ancient spelling. δ in ancient times (until around 2-1BC) was pronounced as "d". Generally, all words with "δ" swifted their pronounciation to what we have today for δ, but some everyday words didn't. Those words (such as άνδρας) had their spelling changed to express the old pronounciation (with d) and άνδρας became άντρας. But the ancient spelling is still around so you can say either. In verbal speech it's one hundred times more possible to hear άντρας than άνδρας, though.