Those who have an educational background in computer science or IT or computer engineering can enter the field of ethical hacking.

They must know programming languages like C, C++, Python, Ruby etc. The understanding of widely used operating systems like the various versions of Windows, Linux and Firefox etc.

Some of the known certifications in Ethical Hacking are

CEH (Certified Ethical Hacker)
CHFI ( Computer Hacking Forensic Investigator) offered by EC Council;
CCNA Security (Cisco Certified Network Associate Security);
SCP's SCNS (Security Certified Network Specialist)
SCNP (Security Certified Network Professional);
CISSP (Cerified Information Systems Security Professional) offered by Intl Information System Security certification Consortium and many more.

Certification from SANS institute in the US though expensive is another valuable certification in the field. Courses can be neutral, or vendor (company) specific such as certifications offered by Cisco, Microsoft, Novell etc.