As Ashvin mentions, there is a basic converstion tool on the support site. This is a good place to start as it will pick up a lot of the common configuration translations.
The tool will then identify the configuration sections that are not recognized. If you don't understand how to manually translate these portions, then we can discuss them one at a time.
I've done a lot of simple conversions using this method.
If you have a large config in a mission critical area then you may want to contact a Juniper partner for professional services on this type of conversion. They can be more aware of your exact toplogy and issues than we will here in a public space.