How do I debug using Dev-C++?

First, make sure you are using a project.

Then go to Project Options - Compiler - Linker and set Generate debugging information to "yes", and make sure you are not using any optimization options (they're not good for debug mode). Also check the Parameters tab, make sure you don't have any optimization options (like -O2 or -O3, but -O0 is ok because it means no optimization) or strip option (-s).
After that, do a full rebuild (Ctrl-F11), then set breakpoint(s) where you want the debugger to stop (otherwise it will just run the program). To set a breakpoint on a line, just click on the gutter (the gray band on the left), or press Ctrl-F5.

Now you are ready to launch the debugger, by pressing F8 or clicking the debug button. If everything goes well, the program will start, and then stop at the first breakpoint. Then you can step through the code, entering function calls, by pressing Shift-F7 or the "step into" button, or stepping over the function calls, by pressing F7 or the "next step" button. You can press Ctrl-F7 or the "continue" button to continue execution till the next breakpoint. At any time, you can add or remove breakpoints.

When the program stopped at a breakpoint and you are stepping through the code, you can display the values of various variables in your program by putting your mouse over them, or you can display variables and expressions by pressing F4 or the "add watch" button and typing the expression.

For more information refer to the help included with Dev-C++.