On the Betfair Developer Program (BDP) forum there is a lot of help for those starting out on coding their own software. With Betfair now migrating from the old SOAP based API-6.0 to the JSON based API-NG I recommend that users read the API-NG forum on the Betfair Developer Program website. My initial thoughts are that API-NG is a vast improvement over API-6.0.
Betfair has now replaced its old SOAP based API-6.0 with API-NG, based on JSON. My first impressions are that this is a much easier API to understand and code to. I have coded API-NG applications quite easily by myself without help from others, unlike the previous API.
Here are some tutorials that I have created to help you on your way.
Betfair API-NG Tutorial #1 - Getting an application key
Betfair API-NG Tutorial #2 - Logging in to Betfair
Craft Betfair JSON Requests with the Aid of Google Chrome
Betfair API-NG Tutorial #3 - Getting a List of Markets (part 1)
More to come.
You might like to consider buying Automatic Exchange Betting, which covers many aspects of trading on Betfair and programming using Betfair's previous API. Although the programming will be different compared to the new API there is still plenty to read about creating your own trading applications.
Automatic Exchange Betting covers the construction of an automated system and goes a long way into system building using odds lines, weight of money, average prices etc. The book is very detailed and will give the reader a deep understanding of automated exchange trading and program development.
Another book that details the construction of an automated betting system is Calculated Bets: Computers, Gambling and Mathematical Modeling to Win. Written by Professor Steven Skiena, Calculated Bets details the professor's successful modelling Jai Alai matches, developing a trading strategy using Monte Carlo methods and then implementing bet automation.
Designing your trading software
Rather than trying to code the ultimate trading platform straight away, you should first build simple single-task tools to handle repetitve tasks. You can then add your own metrics for manipulating Betfair data. After that you might want to automate betting and position tracking. Finally, you can code a bot that does all of the above whilst acting under your own trading rules.
My own trading system is a hybrid, with automation of many tasks but the final decision of when to trade is with me.