I've read some information in Babypips for months, and also some trading instruction/guide in some brokers' website.
I just don't know when to start? How do I know I am ready?
In forex trading, learning continues for ever for lifetime as the market evolves and so there is no 100% guarantee that you are ready to start trading at any specific point in time.
If you have a basic understanding about MT4 usage and forex market and already have a working and proven trading strategy. Then, start trading in a demo account and if you consistently make profit for 6 months, then there are some chances that you are somewhat ready, but it is not a guarantee that you will make profit in real account.
But without demo performance, if you moving to live account, then there is a 99% chance that you will overall loose money over time in forex trading.
But if you don't have a strategy, then try to study support and resistance or trend lines etc to develop your own strategy and then, start trading in demo account.