I think you mean, in what order do elements undergo fusion on the sun. Fission occurs when larger elements, such as uranium, split into smaller elements and release one or more sub-atomic particles such as neutrons, in the process.
Fusion is the joining together of two small elements to form a new larger element. The primary fusion process on the sun is the combining of two hydrogen atoms to form an atom of helium.
On stars like our sun, the only fusion reactions are hydrogen combining to form helium.
On stars that are bigger than the sun, additional fusion reactions can take place. In turn, helium and hydrogen can combine to form lithium. Two helium can combine to form berylium and so on. This process can continue until iron (element 26) is formed.
Fission,in first order burns out of hydrogen,through the high preassure and a lot of heat,elemets are forced to fission,like hydrogen wich fuses to form helium: Remember the hydrogen atom has only one electron joing another hydrogen they share now the same orbital with two electons. So 1hydrogen has one electron+1 electron of the other atom would make = 2 electrons,as we know helium has 2 electons.This process goes on till Iron is reached (wich i really dissbelive,i mean did anyone see that iron is the last member of fussion). So when Iron is reached it cant be fussioned anymore and the sun collapses into its own weight causing a black hole,depends on the suns mass.And one more information,Through fission enourmas radiation is relased.