What brand and model?
Also mAh is how much current you have available (juice to power your gun) while voltage is the electrical potential difference (how much push you can push current through your wires). C discharge rating is something else that is kind of important but just make sure to keep it at or under 20C discharge if using a 7.4V and under 20C if using an 11.1V.
I recommend 9.6V NiMh/NiCd or a 7.4V Lipo if you're just starting out. Also as much capacity as you can fit into whatever space you have. This will depend on the model and where your battery is stored (eg, buffer tube, PEQ box, Crane stock, handguard, etc.)
If you want to get fancy you can even go with a LFP (LiFePo4) battery which is more or less equivalent to a 9.6V NiMh battery.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE