First of all, you should check twitter of your favorite idol as they normally do not have their own homepage and even if they have, it's not updated so frequently and the latest information should be anyway confirmed at their tweet.
Generally you'd better reserve your ticket by replying to the group account or your favorite member account. Otherwise, through the reservation form they designated.
Ticket price will be discounted for reservation and thus it's recommended. At the check in counter of the livehouse, you should tell the name of idol you reserved. Then your paid fee will be allocated to the idol's revenue. It's very important.
As the livehouse is operating as bar, they will take charge for a drink, mostly 600 yen. The clerk will give you a ticket for drink and you can get your drink at the bar counter inside. Various alcohol and soft drinks are available.
Most of the idol live is called "Taiban" style and many idols will play short set list for their allocated timeframe, mostly around 15 minutes.
So you should check the time table for what time your idol will play. Otherwise, you'll miss the stage!
After the live, please take Polaroid (it's called "Cheki") with the idols, as it's the primary source of revenue. Normally one shot with a member costs 1000 yen. During the idol writing the date, venue and usually your name, you can talk with the idol.
This is Yumemy-chan of Candye Syrup @Milkyway, Shibuya, 25th Apr 2019