Bicycle Mechanics - Clank sound

Hello,
'94 Mongoose Hybrid
Clanking sound coming from what sounds like around the crank every rotation of my pedals. It happens twice with each rotation. I know I need to adjust something, but have no idea what. Of course that is the reason I am here! Thanks!

Grab the crank arms and try to push and pull them towards the frame. If there is considerable movement, you have an issue with your bottom bracket or very loose crank arms. Either will require a trip to your local bike shop unless you want to begin the fun path of learning how to work on your own bike. A loose bottom bracket requires several specialized tools to service. Loose crank arms mean you need a new crankset as the arms are probably ruined at this point from riding with them loose. Again you'll need a few tools that you probably don't yet own to do the job.

If your crank seems tight, check to see if your chainrings (the toothed rings on the right crank arm) are tight. If they are loose, tightening requires only a 5mm hex key and flat blade screw driver to counterhold the rear nut.

If the bike only makes the clanking noise when using a certain chainring it could be a simply front derailler adjustment needed. Also make sure the cable from the front derailler is not sticking out and hitting the crank arm. This can make an annoying noise too (and is a simple fix - either trim the cable or bend it out of the way).

Plenty of other things are possible too but doing some more investigation on your own will help pinpoint the problem much quicker than me guessing blindly.

A loose bottom bracket is a definite possibility, one very definitive test is to jiggle either arm sideways and see if you can see movement on the opposite crank arm.

probably the most common problem that manifests like yours is a loose crank arm on the spindle. Idf only slightly loose they click or creak, in more extreme cases there's more movement and more noise, and it occurs twice per crank turn as the rising arm is pushed back be the back leg, then when it comes over the top is pushed forward by the (now) front leg.

Test for this by pushing both cranks to the floor at the same time, then gently backpedaling and pushing both down again. If a crank is loose you'll see or feel movement.