Posting to index

The default behavior of Wp is to put up the latest posts regardless of category. How many is determined by you in admin. In order to change that you need to edit the php call in the loop to call a single cat only or other combo of choice.. It is not for the fainthearted. Details are in the codex.

This is not an answer for which laymans terms are likely to be sufficient. You need to alter the php code. It can be fiddly and it can effect how the rest of your blog works.