The first thing I would do is review the Upgrade road map here http://www.magentocommerce.com/knowledge-base/entry/ce18-upgrade-roadmap
After that you should test your upgrade using this method http://www.magentocommerce.com/wiki/1_-_installation_and_configuration/manual_upgrade_using_fresh_install_and_original_database
The basic steps are as follows:
Back up your current database
Download version 1.9 and run this again your backed up database
Note any errors.
If your upgrade completes then you will be able to login to your admin and review your orders, customer, products and categories
Once you can do this you can start adding in modules (You will most likely have to upgrade some of your modules)
You will then need to resolve your theme problems.
You should be able to get through your upgrade and have to incrementally upgrade. I have personally upgraded many sites as far back as 1.2 -> current version. The exception to this upgrade is going to 1.13. You will need to incrementally upgrade for EE going to current versions. You should not have to do this for CE