Setting Roles and permissions

I am trying to set a role to a user which lets him create a new lead but not edit the existing ones.
So right now i let system manager, and user have access to read and write at level 0, and give the user just read access at leave 1. How do i make this read and create a lead.
ps: there are few buttons in the lead form which makes me give him write access to the user as well at level zero.