read about Mythic affix descriptions in the Codex > Dictionary. each of them have a recipe of 4 Mythstones that are placed in Sockets to make a Mythic affix.
consider if a Mythic is good for the build you are thinking about, or you will end up having to start over with a new item with a different mythic affix. some mythic affixes are good with almost any build, and others only certain types of builds. plus there is how you see your build working with any particular Mythic affix.
last I looked, Storm has a 7 second cooldown, so having enough Cooldown to have it at 5 second cooldown or a little lower is good… ok, looking at the Storm info, putting skill points into Storm increases DMG +5% per Rank, Hit Frequency +2.5%, -1.5% Cooldown, and Elemental Critical (+0.5% I think).
at Rank 20 you get -
+100% Storm DMG
+50% Hit Frequency (each enemy gets hit 6 times a second for 5 seconds unless they die early)
-30% Cooldown (4.9 second cooldown)
+10% Elemental Critical.