Can i Enter a field of biology after I have completed my B.Tech. in Computer Science? which biology field would be best for it?

Entering biology will be a bit difficult after B.tech, surely you can consider taking up a specialization in bioinformatics at masters level.

There are options like M.Tech., M.E, MBA, MS that you can consider.

For Indian colleges offering these courses you can log on to http://www.youngbuzz.com/colleges-guide

FOr study abroad options you can log on to http://www.youngbuzz.com/studyabroad

This will surely help.