Well, there is no real right or wrong way to shape your nails. Some shapes are more stable than others. Personally, I have my nails rounded, this is partly because of the kind of work I do and how my nails grow. I love the look of a square nail, however I can only get them to last about 3 weeks before the corners break off. Then I end up filing back down to a round tip and starting over.
Another way to determine what shape to do is by looking at your nail beds. A rounder nail bed looks better with a rounded tip, a flatter bed works with squared edges. There are quite a few options to pick from, if you can't decide put two shapes together.
First let the nail to grow out straight and then file straight across at right angles with the rest of the nail bed. This is a strong nail shape but is not very flattering. Short square nails can make hands look "stubby". This is best if you have extremely long nails.
Just like the oval shape but square off the tip, this gives the nail tip strength over the oval shape.
File your nail tip to an egg shape and file the sides in a bit. However, this style does weaken your nails some. This is best for nails that are thick or wide- it gives a more feminine appearance.
Great for shorter nails, the nail grows out straight at the sides and then the tip is filed into a rounded shape. This is a good choice to keep your nails strong and short.
Nails are filed away to a point; this shape is very weak because the sides of the nail are totally lost.