Powering Arduino and servo motor
You can get good regulators for this from RC plane sources.  It may be called a "battery eliminator circuit" or BEC.  Just get one that's rated for more than 12V (or 4 LiPo cells).

Connect the grounds so that the Arduino and the BEC input go to the battery connector separately, and try to make the signal wire from the Arduino travel with the servo ground (even if that's not how it's shown in the schematic).







simulate this circuit &ndash; Schematic created using CircuitLab