First please check the band's website for ticket instructions. Usually you can send an email or direct message to the band's public account. Please give your name (first name alone is usually fine) and how many tickets you wish to reserve.
Please note that you almost always have to pay an extra ¥400-¥600 as a mandatory drink fee in addition to ticket price. For example ticket is listed at ¥2000, drink fee is ¥500, you pay ¥2500.
Doors usually open 30 minutes before the show starts and shows start on time. When you arrive, if you reserved a ticket beforehand (by directly contacting the band) please tell the person at the ticket counter 1) the name of the band you contacted and 2) the name you gave when reserving the ticket. You will then receive your ticket and drink ticket.
Take your drink ticket to the bar and hand it to the bartender when you order. Depending on the live house, drinks may vary but they always have beer of some sort and other basic drinks.
Photos/video may or may not be allowed. Use your judgment. Be nice. Have fun.