Total noob question.

[Atrocities]

Hello,

If the AI declares war upon you the chances of making peace with them is virtually impossible. It has been a long standing recommendation to new players in the event that they wish to make peace with a waring AI race to employ the Gun Boat method of peace.

In other words, you kick his arse and demand his surrender until it is given. Good luck and enjoy.

Recommended Reading:
E.R.A.S.E.
Insights To War

[Tim_Ward]

You have to set "Turns Since Last War Before Friendly Treaty" to 0 in the AI politics file. Then they'll make peace.