Migration tool comes with the installation.
You can use db 11g for the backend, but it must be at least 22.214.171.124.
Before doing any application upgrading, I strongly recommend that you read the documentation, specifically the Forms 6 to 12 Upgrade Guide. Even if your old app is currently based on a version newer than 6.0.x, this document will apply. In most cases, using the FMA (Forms Migration Assistant) is unnecessary and often overkill. Making any needed changes manually will often render more preferable results.