[Django]-How can I protect my web-based game against cheaters?

SQL Injection

Prevent malicious users from altering SQL queries via URL query strings.

DoS Attacks

Prevent users from the same IP address from accessing your site an excessive number of times in a small space of time.

Password Strength

When allowing users to create their own passwords, show a password strength indicator which encourages users to enter stronger passwords.

Captcha

Stop non-human users from submitting to forms by presenting a captcha image. You may also want to use this if password authentication is failed multiple times, to prevent robots from guessing passwords.

The Google Browser Security Handbook has a lot of information about potential vulnerabilities in the web architecture, in particular the details that are affected by the behavior of web browsers (as opposed to server based vulnerabilities, like SQL injection attacks and the like). It is a good starting point for learning about how browsers work in ways that impact security, like how they handle cookies, cross domain requests, images and MIME types, etc.

One book I might recommend is “Security Engineering” by Ross Anderson. It’s fairly detailed and it gives a good overview of many different topics relating to computer security, although not all of it is relevant for securing a website.