There are tons of tutorials there.
One of the best http://forum.mikrotik.com/t/advanced-routing-failover-without-scripting/136599/1
and this on the wiki page https://help.mikrotik.com/docs/display/ROS/Failover
Please start to use them and play around and then report back.
Once you understand what is going on, it will be easy for you to assign in the mangle rules devices or IP ranges or bridges or VLAN to specific ISPs.