Government & Defence Entrance Exam

Yes, you are eligible for SSB.

You can appear for NDA after 12th or CDS after graduation. Along with your academic study, you need to study GS and GK of NDA exam. Regular study of two year is enough to crack NDA. You can also appear for CDS in final year of graduation. OTA give training for Short Service Commission for CDS qualifier. Joining Indian Army through NDA has many advantages.

You can take online coaching from career Launcher, Gradeup etc. It is not necessary but will make you prepare a think from different angle. Thus will definitely add value to your preparation. Study along with coaching can do miracle. But without self study you can not clear it. Best wishes.

You can appear for IAS with your commerce higher study. Better it would be if you are more clear with your goal. IAS if you love serving people and could work 24 7
IAS is prestige and pride. If you want to expertise in accounting go for CA. It is almost a life time profession and it would be more lucrative career if you can do LL.B. Along with CA. Best wishes.