There is an abundance of information answering all of your questions in this site if you look for it.
1. To commission after college you must apply for and be accepted to OCS. After finishing OCS you will be commissioned in the designator you applied for.
2. The test you take is either the OAR for most designations or the full ASTB for aviation. The OAR contains math, reading, and physics questions that are not dissimilar from the SAT. The ASTB encompass the OAR but also adds other elements specific to aviation as well as a performance based measures test. All this information can be found on this site.
3. I would not say that getting in as an officer is easier. There are much fewer spots available and higher standards to qualify. The designator that you apply for will determine what prerequisite GPA and test scores you need as well as other requirements. Some designators weigh test scores more, while others may weigh GPA more. Look up the program authorizations on Navy HR to find out what you can qualify for. At the end of the day though, higher scores and GPA make you more competitive, but everything is depending on the needs of the navy.