Unfortunately there is no simple way to migrate from a custom Magento 1 store to Magento 2. There are a lot of tools available to help you including the official migration pland and tool which you find more information from the official documentation here
The migration tool helps you to migrate Magento 1 store data such as customers, products, categories etc. Unfortunately the tool does not help you to migrate your custom theme or modules to Magento 2.
You will need to consider your own migration plan which should include
- Finding a new Magento 2 theme that meets your requirements
- Auditing your Magento 1 commercial modules and checking for Magento 2 versions
- Auditing your Magento 1 custom module code and investigating how to develop this functionality in Magento 2
- Building your Magento 2 development environment for testing
- Testing the migration tool to see if your custom Magento 1 data is compatible with the tool
If you are unable to do all this work yourself there are plenty of Magento agencies and developers that have a lot of experience with Magento 1 migration who will be happy to assist you.