Unless your subject is sitting you'll mostly want to use the vertical format.
Don't forget the guidelines for good composition, try and place an eye on a ROT power point.
Direct sunlight is BAD. Avoid it, and dappled light at all costs, using open shade instead.
Hopefully you have looked at some online info about lighting portraiture. There is short lighting, broad lighting, rim lighting, butterfly lighting, and others.
The right location having open shade allows using sunlight as the main light and a low power pop of on-camera flash, or light from a reflector as fill.
Focus on the eyes. They are the most important part of a portrait and at least the eye closest to the camera needs to be tack sharp. Having both eyes tack sharp is even better, so be carefull with depth-of-field.
Posing is another portraiture skill. avoid the football player look with the shoulders parallel to the image sensor. Visible joints should be bent. Don't frame a shot so a limb is cut at a joint because it looks like it was amputated.
I want my subject to stand or sit tall, leaning from the waist just over where their belt buckle would be and most of their weight on the back foot.
Don't let any nose shadow intersect the upper lip line.