The default is the most recently published post appears first, and goes backwards from there.
1) on the list of posts page, click the post names and then click Quick Edit – check the publication date – make sure the dates, especially years, are set correctly. If not, you can change them there.
2) If those seem correct, temporarily change your theme to the default theme (Settings / Appearance, and activate default theme). The reason for this is to see if the problem is just with your theme or across the board. If they sort correctly on the default theme and not on your real theme, there is some code in your real theme causing this.
After you test, switch back to the real theme. If the problem is only in your theme, post that information and someone will help you locate the code that needs to change.
