As a KRL regular passenger, I can provide some good tips if you want to ride KRL around Jakarta.
If you have an Android smartphone, you can download the Info KRL to track the train position and the estimation time of arrival (It's quite accurate!). It also has a fare calculator to calculate the fare between the stations (Recently they switched to distance based fare, so this feature will be useful if you don't know the distance between the stations)
Although if you are not a regular passenger, I suggest you to buy bank card (I recommend you to use BCA Flazz). This will come in handy if you want to switch to Transjakarta (Both accept bank cards) or when using another public transportation in another city in Indonesia, such as Trans Jogja.
Do not ride around 6-8 am and 4-8 pm if you don't want to get a super crowded train, especially on Yellow Line (Sudirman-Bogor Line).
If you are forced to ride these crowded train, pay attention to your belongings. Some pickpocket uses razor blade to cut through your bag and steals your phone, wallet, etc. inside. Put them deep inside the bag so they can't be reached by those pickpockets