The written/computer test portion is one of the most important components of each application. It's essential to do well not only to avoid disqualifying yourself, but to create a more competitive application. Instead of branches, the information below will be broken down by test.
ASVAB/AFQT
ASVAB and Army Jobs - provides a breakdown of how each area is scored, and each the minimum required score for each available job (does include enlisted options) - http://www.military.com/join-armed-forces/asvab/asvab-and-army-jobs.html
Official ASVAB Testing Page for Applicants - complete walkthrough of the test, how its scored, what to expect, as well as sample questions - http://official-asvab.com/applicants.htm
Free Practice ASVAB Exams - can choose which sections to include, large bank of questions pulled from - https://www.4tests.com/asvab
Practice Exam - explanation of each section as well as a practice portion for each section - https://uniontestprep.com/asvab/practice-test
10 Free ASVAB Practice Tests - http://www.asvabpracticetests.com/
ASTB/OAR
ASTB Overview - explanation of subtests, scoring, general information - http://www.med.navy.mil/sites/nmotc/nami/Pages/ASTBOverview.aspx
Air Warriors ASTB forum - user forum with references, place to ask and look over questions, and links for any and all ASTB information - https://www.airwarriors.com/community/index.php?forums/astb.47/
There are also numerous study guide books that can be purchased in stores or online. A popular one is here - http://www.amazon.com/ASTB-Study-Guide-Practice-Questions/dp/1941743447
PDF Study Guide - study supplement with practice questions - https://manlypat.files.wordpress.com/2009/07/astb_study_guide.pdf
AFOQT
AFOQT Study Guides - comprehensive all-in-one website with practice exams, study guides, and a detailed explanation of the test itself - http://afoqtguide.com/
How to Prepare for the AFOQT - another detailed breakdown of the exam, with several links to popular physical book study guides, including the most up to date ones - http://www.baseops.net/afoqt