Use either no nails glue to fix the skirting or screws and rawl plugs. To join corners you can either mitre on 45 degree angles on corner to fit or scribe it over which is basically fitting one piece square to the corner of the wall and then cutting the edge of the other piece to the shape of the skirting board so that it fits over the piece you have fit squarely on to the wall. Hope this helps and you can understand it, it is quite difficult to explain.
If you use screws or nails, you'll have to make sure they go through the skirting and into the wall studs. (The upright pieces of wood that the plasterboard, or lath and plaster, is fixed to). Screwing or nailing to any other place on the wall won't properly secure the skirting.
If it is a solid wall that hasn't had plaster board fixed to it, it won't matter where the screws go. Nails should only be used if the walls are breeze block or thermalite block.
I reckon its best to scribe it the way kaztuz mentions above because although mitres can be cut accurately to 45, the walls that they butt against are rarely 90 degrees, leaving gaps.
The right way to fit skirting boards is to start on the wall facing you as you walk in the door,this length would have a square cut at each end and you would then work back towards the door from both sides, scribing the joints as you go. All internal joints should be scribed and all external joints obviously have to be mitred. You do not need to over fix the skirtings, 2 srews & plugs or nails or even a good adesive such as gripfil every 700-800mm is sufficient.