How do I attach power amp to 1832 mixer - Behringer Xenyx 1832fx Mixer 18 Input 32 Bus with Efx USB

There are a number of configurations for a mixer like the 1832, the most common are PA system and Recording system. There are Main output jacks which carry the mix as summed in the main faders. These in a PA system would feed the pa amplifiers which drive the house speaker system for the audience. These connectors are XLR 3 pin type on the rear, intended for professional pa amplifiers which use balanced +4dbv line input level. Amplifier which have 3 pin XLR connectors for input signals can accept that high level signal. For home style HiFi amplifiers, the signal level is nominally -10DBv and balanced or unbalanced signal lines with 1/4in diameter phone plugs which are connected to the mixer by way of a second set of Main output connectors which are 1/4in phone jacks. Use whichever amplifier input level your amp has.
If you are not using it as a main PA house mixer, but using it for recording, the main output connectors go to a 2 channel recorder or computer sound card recorder. The power amp and your monitor speakers are connected to the connectors labeled "Control Room" (CTRL Rm). These output are useful for recording because the signal through the mixer can be monitored from several buses, Solo, main mix, selected by buttons provided. The buttons only affect the signal heard through the control room monitors, and not the main output which only sees the main mix so selecting "solo" does not interrupt the signal going to the recorder while the engineer in the control can be checking other signal paths through the board.