Clean up PHP / Secure & Make pretty

I have created a script myself using what I know to get it in a working order. I now need that code looked at, tweaked and cleaned up. The points I need to touch on are changing all the queries to MySQLi and revamping the login / session part to make it more secure. Everything that I have done is working. I need it to work how it currently does but get it cleaned up. If there are places you can create functions instead, please do so. Because this is already written, please don't bid a crazy amount. All i need is to take what I have and make it safer, and cleaner.