I got my first water bill
On a bill I am used to getting bank account number into which I need to pay money due. I then usually can go and pay online, however I couldn't find any bank numbers on the slip...
How does one pay water bill in Thailand?
Many condo buildings allow basic utilities (but not always electricity) to be paid at the condo management office. Looking at your bill, it looks like this is how you should pay this bill.
Most bigger stores will have a counter were you pay bills and, often, a money exchange, where you can load cards it sells. Many banks adjacent to their ATMs have a bill pay online; it is the machine door that opens up & you put cash into to pay. It's like an ATM, select bill pay on the screen above and it takes cash. You can also go to a bank and buy a prepaid debit card. You put money on it, then use the ATM machine at the bank to pay bills.
All you need do is ask around; someone will tell you where to pay close to where you live. It is the Asian way of doing things. In Thailand, you'll find these machines where you can pay online in some 7-Elevens, almost all malls, and most banks.