The volcanism feature of EDDB just gives you an additional column where you get information of (i) if the planet has volcanism and (ii) what kind of volcanism. If a planet does not have volcanism, it will not have geological sites. Do not go to those planets. Pick a planet with volcanism.
On a planet with volcanism, you will find geological sites. Those sites will be denser in material drops and (even more importantly) have a number of high grade drops in formations called "Needle crystals". There are four types of formations at a geological site - from pieceous cobble (lowest grade) to needle crystal (highest grade). If you go to the planet entry at EDDB, you can see what materials it drops from each grade formation. For example, this is the material content of Alpha Centauri B1:
The materials in blue will drop from pieceous cobbles, polonium will drop from needle crystals. The green and yellow will drop in the two middle categories, respectively. If you want a particular material, you should search for a planet where that material has a relatively high % in comparison to the other materials in the same category. For example, this planet will drop iron from pieceous cobbles a fraction 35.2/(35.2+26.6) = 0.57 of the times and nickel the other 0.43 times. Each planet tends to have only one high grade (red) material, meaning that all needle crystals will drop that material.
Edit: Selenium is special. Dropping together with the green materials and being a G4 material itself. It is difficult to find a good source as other materials in the same category generally have higher percentages. You can either cross trade for it or find a special planet such as LHS 417 9 E A:
This planet (actually, moon of a moon) will drop selenium about 2 out of 3 times in the correct formations.