Re: what are the first ten most common elements?

Alex, this sort of information is not hard to find, once you have decided on exactly what your question 
is.

Here are your problems: 
(1) Do you want to measure which is 'most common' by atoms or by mass. An iron atom is about 56 
times as heavy as a hydrogen atom. so if you had 10 kg or iron and 1 kg of hydrogen, you have more 
iron by weight, but more hydrogen by number of atoms.

(2) When you ask 'most common' it is important to know where you are talking about. You might mean
   a) In your kitchen
   b) in the human body
   c) in the biosphere (all living and recently dead things on earth)
   d) in the atmosphere (air)
   e) in the hydrosphere (oceans)
   f) in the lithosphere (crust of the Earth -- surface rocks and minerals)
   g) in the Earth as a whole
   h) in the solar system as a whole
   i) in the universe as a whole

To find the answer (depending on which one), almost any textbook on geology or environmental 
science might be able to help you. You could also check references like the CRC Handbook of 
Earth Sciences and Physics.

The most often quoted result is for the most common elements by mass in the lithosphere. In order, 
they are:

oxygen, silicon, aluminium, iron, calcium, sodium, potassium, magnesium (different sources have these 
last four in different orders), titanium, and hydrogen.

For the earth as a whole, the order is changed because magnesium is much more abundant in the 
mantle (along with silicon, oxygen, and potassium), and iron and nickel are the main elements in the 
core. 

For the universe as a whole, the common elements are recognized as hydrogen, helium, oxygen, 
carbon, neon, nitrogen, magnesium, silicon, sulfur, iron. There is a lot of uncertainty about the 
composition of the interior of the sun, that makes it difficult to put the elements in order reliably for 
the solar system. The most common elements are still the same ten, but some people think that heavier 
elements like iron and silicon are much more abundant in the sun's interior, and therefore in the solar 
system.

For the oceans, oxygen and hydrogen are obviously the most common elements in that order, because 
the oceans are made of water mostly! After those two elements, we come to the salts, and the next 
eight most common elements are chlorine, sodium, magnesium, sulfur, calcium, potassium, bromine, 
and carbon.

In the atmosphere, nitrogen is most common, followed by oxygen, argon, hydrogen (in water vapour), 
carbon, neon, helium, krypton, xenon, sulfur, and chlorine. (but the last three are much less than 1 part 
in a million)

You can find some of the other orders at this website