This is not a simple journey, and depending on your size, you will require IT support services to migrate your organisation's data. Ensure you have a provider that is familiar with the Google environment, and has supported migrations previously. If you are looking for a provider to support, you can access the Connecting Up Supplier directory to search for specialised NFP IT Provider, and this guide will support you to assess potential IT providers. 

As you are a charity you may qualify for the Connecting Up services and discounts, including Google Nonprofit licenses.  This guide talks through accessing the Google for Nonprofits discount program.