Hello there,

Yes you join army immediately after 12th if you are from arts / humanities. You can't join IAF and Navy because they ask for PCM.

You can apply for NDA ( National Defence Academy) conducted by UPSC twice a year. Once you clear written exam , you will get call letter from SSB after clearing . You will get an opportunity for training at NDA khadakwasla Pune. There you will study for 3 years in the opted discipline and it may be arts .

Good luck . Chase your dream.