Security testing is very wide subject which cannot explain in single word. Also deals with so many factors.
1. Network security
2. Web security
3. Application security
4. Database security and so many.
All above categories contains so many methods, tick to check security area. There are several tools; technologies are available to test different security threats, area. It is better to search through Google or read past posts in this forum.
Security is a very broad topic that covers a lot of aspect.
In addition to those mentioned by priyank1985, there are also those that must be taken into consideration, such as:
and many more