Positions are extremely important in Warframe. Melee combat is incredibly powerful and your stance will dictate your available movement set with whatever melee weapon you use. This means that the actual combos you can do, and their effects, are all based on your location. Most weapon types have multiple stance mods available to them, and you can change the style of any weapon by changing stance.
This means that it's a good idea to experiment with positions and combos to find something you like that is effective.
How to check position combos
You can see all the combos that will be available with a stance by clicking the "Combos" button next to the stance mod when installed in a weapon.
How to get position mods
Stance mods come from a range of in-game enemies, various bosses, bounty rewards, or Conclave. You can also purchase valuable stance mods from Baro Ki'Teer from time to time.
If you really want a position you have to keep in mind that all positions can be traded between players and the trade tax is normally quite low. You can use a website like Warframe Marketplace to find someone selling a Stance you want, but you will normally have to part with a Platinum to get it.
You can also use the in-game trading chat to request an item, and if someone in the chat has it, they may be willing to sell it to you. It's always a good idea to use the Warframe Market to get a good idea of the mod's current rate before you buy.
If you prefer farming for mods, you can find a full list of in-game positions, and a few
