The cost of a local bus is $7.5 MXP. There are no transfers. Buses run until about 11 or midnight. There is a Night Owl that runs every hour with a yellow flashing light on top - it is $15 MXP. Have exact change.
Give money to the driver and they will give change. But it is best to use small bills (don't give a $500!). If you are boarding with more than one person, you can pay for multiple people.
The driver will give you a ticket. Hold onto this as an inspector might come on board and ask to see it.
Press a button located near the exit about 2 blocks before you are ready to get off. Exit through the back if the bus is busy.