increase posts

Hi there,
Could someone please help me? I am trying to increase the number of posts on my blog from 10 to greater say 15.
I have searched google and forums, but even when I have come across the same problem, I have had trouble understanding the answer.
Could someone please let me know in simple laymans terms, what I can do.
I understand it is coded into the theme, I have search for what seems like days now, and cannot work out from where.
thanking you in advance.