Do you want to begin with C(++) or do you want to begin to use your knowledge of C(++) with MorphOS.

For the 1st, start with some "for dummies" style of tutorial from the web, plenty to choose, just make sure they don't steer into Windows specific stuff.

For the later, take the examples in the SDK and experiment.

At some point you may want to go into GUI and you'd have to choose to either use MUI which is optimal for MorphOS but comes with small compromises for other Amiga systems or you go down the route of using archaic stuff like GadTools which delivers the same level of "not so good" over all systems.