To make yourself admin, you need to install AmxModx on your Counter Strike 1.6 Steam or Non Steam Server.
You Can find complete tutorial here How to make CS 1.6 Server and install AmxModx
After installing amxmodx, follow these steps.
Go to HLDS/Cstrike/amxmodx/config and open users.ini file.
Scroll to the bottom of the file and there you will see something like this.
You can make yourself admin with your name, ip address or your steam id. For example i want to make myself admin with my name, then i will write my name and set proper flags as shown in picture. You have to write your name and your password in place of "my setinfo password" and save the file.
Restart your server or change map and your done. Now open your game, write in console setinfo _pw yourpassword and hit enter, join your server.
You can set user accoun flags according to your choice. Flag "a" means that you require password to enter in the server. Flag "e" means that you dont need to enter the password, but it is not secure as anyone can use admin nick and enter in the server.