A plumbing leak at a dishwasher line is just like any other water supply line to a fixture; it’s just typically a longer line because if goes from under your sink all the way to the dishwasher.
You will find these water supply lines in a variety of materials from plastic to copper to braided lines. Braided lines have female threads on them and they simply screw on to the angle valve under the sink. For the lower cost options, they use compression fittings to attach.
Slide a nut onto the line, then push on a ferrule. Mate the line into the end of the angle valve opening and seat the ferrule at the opening angle valve. Thread the nut onto the angle valve and tighten it until snug. The nut will compress the ferrule and seal out any leaks.
Finding a leak is really about running the water and looking around. A leak will typically be located at a connection where a water supply line meets a valve or the machine itself, but this is a general statement. There is also a possibility that the line itself is damaged or even the dishwasher may be leaking. You really have to troubleshoot it and not assume it can only be one problem.