First things first: be certain of your monetization strategy! At the end of the day you are looking for a return on your investment and if you do not know the value of your users you will not be able to make informed decisions on UA.
Secondly look at your goals for a campaign. Is it sheer numbers or are you looking to bring in a higher quality (engagement, retention, etc) user. If the former - i.e. burst campaign - look into cheaper sources that offer scale of installs - though many of these will be incentivized and not of quality to your DAU count, the scale offered is very strong. If looking for higher quality focus on Facebook for its targeting capabilities, as well as video providers such as Vungle, AdColony etc. These providers will be able to deliver a much higher quality user - though at smaller numbers. These networks are much better suited for sustained buys that can take place over the long term.
Finally, know how you plan on paying and tracking. You will need some form of tracking solution - either a third party like MAT, a server to server, or the ad network's own SDK - installed if you plan on being able to track / pay on a Cost per Install basis. If not you will have to run cost per (completed) view or cost per click which can be riskier.
The basics are given in order of importance and best of luck, mobile advertising can be a tricky space to navigate when you are first entering!