Go to a shore, dock or bridge. Both freshwater and saltwater crabs enjoy areas of cover that allow them to feel safe. They will hide below rocks and in the shade of a dock or bridge. These are the places you will most easily capture crabs.
Use a crab net. To capture a crab with a net, wade out to the shoreline. Be sure to move slowly to avoid spooking any crabs away. Wait for a crab to pass near, then dip your net down and around the crab. Lift the crab from the water and place it into a bucket or cooler.
Use a crab trap. Alternatively, if you do not have the patience to wait for a crab to wander by, you can use a crab trap. Purchase a trap from a fishing store. Bait the trap with chicken or a specialty bait. Secure the trap with a sturdy line to your boat or dock. When a crab enters the trap to eat, pull the line tight to lower the door and trap the crab. Remove the trap from the water and place the crab in your bucket or cooler.
Watch the crab in an enclosed environment. How you study a crab will depend upon what you want to learn. If you are interested in how a crab reacts to outside stimuli, watch the crab and note what it does in different situations.
Dissect the crab. If your interest lies in the interior parts of a crab, you will want to dissect it. Crab dissection is a common activity in schools, and there are many online guides to the dissection process. Use one with diagrams that explain the different parts of crab and what they do. You will need a scalpel to carefully remove the various sections while keeping the parts intact.