Can I do any computer graduate course after B.Com? Will certificate courses give you good job opportunities?

I would rather suggest you to do SAP FICO for better future. Main reason is that most of the big companies uses SAP as an accounting software.

After completing your graduation with B.Com, you have to learn Accounting softwares such as Tally with ERP. If you have done one year coarse called PGDBM, then it would be superb for you. Best of luck for your future.