As a matter of fact ANY ACCOUNT can be a witness if that account wishes so, however in order to become a successful long term STEEM witness one MUST be in possession and control of a reliable and powerful server located somewhere secure with a stable internet connection. In other words since witnesses "mine blocks" for the Steem blockchain, one must at least have a powerful stable server first capable of mining those blocks to be able to do the job of being a witness. 

Anyway here is a good tutorial link explaining in great detail what is a Witness, what does witnesses do, what are the tools needed and how to set it all up from scratch. Every information needed on how to set up a witness server is in here:

https://steemit.com/witness-category/@klye/the-complete-noob-guide-to-steem-witness-setup

It is also worth to note that since Witnesses uses a server and need to have powerful hardware capable of mining blocks, they have a monthly maintenance fee and subscription for all of these. So if a new witness fails to get into top 50 or even top 100 of all the witnesses, then he will only be able to mine fewer blocks making it totally unprofitable for him/her.