First of all if you are on this forum, i think you already have installed a linux distro on your system.
so here we go step by step :
1. Use port scanner tools like nmap, W3af, Nessus and many more. (These can also be used further not limited to port scanning)
2. Use Nikto to find vulnerabilities on a webserver.
3. Regarding your question about "How to find admin page" If the target website is nothing more than a simple HTML doc then you can find OS of the webserver using nmap (nmap -sV
www.target.com) and try going to cpanel directly by puting port number next to URL in address bar like:
(my school's website is
www.ngfssaket.com) - (In the address bar :
http://www.ngfssaket.com:2082) you are lucky if it works else try using ADMIN FINDER Software. Most easy to use is HAVIJ (Google it and download).
you can use havij for SQL Injection if the website pattern goes like :
www.target.com/anything.php?q=1234 can also be used if the website is in .asp or .aspx
4. About Nikto it is useful in case you are planning to a do a serious attack on your target. It can help you in finding various vulnerabilities including HTTP methods, CGI, Database etc.
5. you can also use NETCAT to do HTTP attacks
COMMAND (Post installation)
nc
www.target.com [PORT] usually the port is 80 i.e., TCP
after that you write OPTIONS / HTTP/1.1
Though OPTIONS method is not counted as a vulnerability but it can help you in finding other lot of vulnerabilities. This is what makes OPTIONS method enabled on a webserver a dangerous thing.
Hope this helped you

Good Luck
Don't do any black hat activities unless you feel comfortable with it.