Make sure you have root access and make sure you have yum, rpm, scp, curl, wget, unzip, tar, yum-utils, createrepo, reposync installed, working and in your path.
Make sure networking, iptables, proxies and firewalls are all open enough for you to access the Hortonworks repos and the bandwidth is decent.
Download the OpenJDK 1.8 64-bit update 51 or higher
Make sure you setup passwordless SSH to all machines, including the current machine you are starting from which is the Ambari Server. You may need to SSH to yourself.
Make sure you have 20G+ of /var space, 20G+ /usr space and plenty of /tmp space. Many things will go here: /usr/hdp
One time I needed to manually install MySQL via sudo yum -y install mysql
Any space where you will install your data nodes will need to be owned by hdfs:hadoop and have 755 access.
Keep community.hortonworks.com in a browser, you can quickly search and find more answers.