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The tutorials and the wiki are always a good starting point. Have a look at this wiki page and this, this and this tutorial.

You always need a message file to generate the C++ files that define the types you need for subscribing and publishing. For instance std_msgs/msg/String.msg is used to generate std_msgs/String.h which is for instance used in the above tutorials. ROS provides a lot of pre-defined messages and if you can, you should use them. If you cannot find a matching message, you need to define it yourself.