can you please tell me the answer
Apart from the excellent advice tedster has already given you, it is probably not possible for anyone to give the overall answer since each subject area tends to vary.
Some subjects are dominated by big money sites spreading themselves all over the place by various means, some by big companies with SEO "experts" in many sectors, some subjects are dominated by comparison/selling sites and some subjects are dominated by real industry experts who have done it all for love and greater trade knowledge to be passed on for free.
I have absolutely no doubt I'm missing out some others as well however if you are just starting at this you had better understand that there are many, many people with a huge amount of experience before you in probably nearly every subject and for a newbie to even consider this task would be daunting.
Perhaps if you told us your/your company intentions and general sector then more useful advice could be given.
For instance is it for you setting-up as an SEO?
Have you been asked to SEO a company site or many?
Are they brochure sites or e-commerce sites?
There are lots and lots of questions even for a generalisation.