So basically you want a “main” WAN and a “failover” one.
The simplest method is (IMHO) this one:
http://forum.mikrotik.com/t/simpler-failover-for-two-gateways-i-found-working/169108/1
further simplified in this post:
http://forum.mikrotik.com/t/simpler-failover-for-two-gateways-i-found-working/169108/1
If you have difficulties in following it, just say so.
A good idea in case of issues would be to post your config, instructions here:
http://forum.mikrotik.com/t/forum-rules/173010/1