If you wish to change permissions for a file or folder (e.g. to make it writeable), there are two methods (the first of which is probably best if you are not familiar with FTP).
1) Login to your cPanel, and click 'File Manager'. This is a web-based front end to your files. Navigate into your public_html/ folder (this is the folder that contains your website). Now, locate the file or folder you need to change the permissions for. Click on the file or folder name, and you will see a list of options on the right hand side, one of which is 'Change permissions'.
2: If you haven't already done so, install an FTP client. This is a program that allows you to login and view/edit your files on the server. We recommend FileZilla (which is an excellent and free FTP client available at http://filezilla.sourceforge.net). Launch FileZilla and create a profile for your site (the FTP information was provided in your Welcome Email). You can now login to your site and navigate your files as above. To change permissions on a file or folder in FileZilla you right-click on it and click 'File Attributes'.