Windows 10, unlike some earlier Windows versions, automatically attached IIS to port 80 when the system starts up. This results in you getting an error message about the port already being in use when you try to start Apache.
The solution is to disable the service that is starting up IIS and attaching it to that port. The port will then be available for use by Apache.
The simplest way to do this is to log in using an account with administrator access. You can then hold down the Window key and press 'R' to open the run dialog. Type 'services.msc' into the field to get access to the services. The service you are looking for has a description of 'World Wide Web Publishing Service'. The easiest way to find it is to sort the list of services by the Status which should move the one you are looking for to the very bottom of the list (or very close to it).
There are now two things you need to do. First right click on that service and select 'Properties'. In the middle of the page you will see a field labelled 'Startup Type'. You need to change that value to 'Disabled' so as to prevent the service starting up the next time you reboot the system. Press the 'Apply' button to apply that change.
The other thing you need to do on that same screen is to press the 'Stop' button to stop the current execution of the service and free up the port for use by Apache.
Now you should be able to start Apache without it telling you that the port is in use.
Note that if you want to use both IIS and Apache then instead of disabling the service as described above you will instead need to update one of the web servers to use a different port.