Hello lejpjj. If you're who I think you are, you've already asked these questions through Live chat, but I'll try to answer them again here for others who might need them

. These answers will pertain to a CS:S server.
lejpjj wrote:1. I want to change the motd, the picture thingy appearing as soon as you have connected to the server.
The motd.txt file is where you can set this. You'll see it as "./motd.txt" through our "File manager" page or through the FTP as "motd.txt" in your "cstrike" folder. The file can have one of two formats:
1. Straight HTML code.
2. A single line that links to a webpage. For instance "
http://yourwebsite.nuclearfallout.net/motd.html".
Of the two, the second is the more flexible solution, because it allows you to specify a larger file.
2. I need to know how I talk "green", so I can tell people to follow the rules if they break them etc. The "green" chat will be seen by everyone.
You can use "rcon say" to show text in the server without a plugin, but it will not be green. For green text, you'll need to use the admin chat option of a plugin such as "mani mod". We have an installer for this plugin on our Easy setup page.
Generally we don't recommend using mani any more since it's rather buggy and no longer supported. It would take a little more work, but a better solution would be to use Sourcemod (which we also have an installer for) and a modified base plugin that makes text green. This thread on the Sourcemod forums talks about that option in more depth:
https://forums.alliedmods.net/showthread.php?t=82409
3. I want a welcome message on my server, that says something like 'Welcome! Please follow our rules. Have a great time!'. And if possible, It would be great to have a box appearing when u join aying a little about the rules, and on it you have an option (press 1. to accept the rules, 2. to not accept the rules.) and if he/she does not accept the rules, the person will be kicked.

That would have to be provided through a plugin as well. One that looks like it might work would be this:
http://addons.eventscripts.com/addons/view/es_readrules, but it would require the installation of a plugin system called EventScripts first. Another good place to find or request such a plugin would be the Sourcemod forums:
https://forums.alliedmods.net/forumdisplay.php?f=52; plugins from this site would require Sourcemod.