I recently installed 7.5 which I think I got up and running in record time, absolutely no problems at all.
To enable xDB via Mongo you will need to follow the configuration guide.
It's fairly stright forward just install Mongo launch a mongod instance specifiying a datapath (mongod --dbpath C:\mongo\data) and Sitecore will do the rest.
What did catch me out (and this is only if you are using MVC - which I hope you are) is the Visitor identification control which needs to be on the page. Obviously we can't use the <sc:VisitorIdentification web forms version.
To add the MVC version which needs to be on every page:
@using Sitecore.Mvc.Analytics.Extensions ... @Html.Sitecore().VisitorIdentification()
Now you should be tracking visits.