You connect your fans to either the 4 pin or 3 pin fan headers on your motherboard.
they look like this: (here they are white, may be black)
Usually you should use the 4 pin headers on 4 pin fans, as they will allow you to contol the fan speed, but some 3 pin headers also do that, use 3 pin headers for 3 pin fans. It should be noted that either plug will fit either header with no issues.
Look for the labels "SYS_FAN", "CPU_FAN" or "CASE_FAN" near those headers.
Alternatively, your case fans might have come with Molex connectors, and you connect them directly to the PSU. If that is the case, I may tell you how to reduce the speed on them, as by default this type will run on 100% speed all the time.
You can also tell us the name of your motherboard so it's easier to point to the correct ones, or you can consult the motherboard's manual.