Here's a video that might help - https://www.youtube.com/watch?v=pCeC9_bHkRM
Did you have one before? I ask because you’ll need an electrical outlet and the special T in the sink drain.
Your new one should come with full instructions.
Call a plumber.
Electric code requires a grounded 15/20 amp line for the disposal as well as combination GFCI/AFCI receptacle that the disposal plugs into even if it is under a cabinet and no matter where the switch is. Speaking of switches, you might want to look into an air switch. You should have a dedicated circuit for the disposal, but you can share a circuit with the dishwasher. Check to see what the manufacturer recommends.