Salting aubergines before cooking is a tradition that originates from times when aubergines were more bitter and therefore this was used to help reduce their bitter flavour.
Many people still salt their aubergines to get rid of excess water. As aubergines are 90% water anyway, I don't find that this makes a huge amount of difference.
If you do want to salt yours then lay them on some paper towel and sprinkle salt over the top. Leave for 5-10 minutes and then press some more paper towel over them to remove extracted liquid.