How I can change permissions on my server?

Changing permissions depends on the type of access to file on your server you have.
Most FTP-clients (cuteFTP, smartFTP) allow you to change permissions by clicking right mouse button and then choosing "properties" or "permissions". If you use FAR manager you'll need to press 'ctrl+a' while cursor is over file or folder to change permissions. If you use shell - you'll need run command 'chmod'. For example:
for folder permissions:
chmod tmp/ 777
for file permissions:
chmod db.conf.php 777
(these files and folders should be located in your current folder)

If you use control panel file manager - choose file or folder, and on the left or right hand panel with permissions will appear.
Also some Windows-based (e.g.: IIS) servers don't support permissions changing (sometimes doesn't have 'chmod'-command at all) but there folders and files mostly have permission 777 by default (Or you’ll need to contact your ISP to change permissions on such server).