How can I match the goods and services that my business provides to a relevant class?

You can use the Trademark ID Manual for this purpose.  A link is provided below:

https://tmidm.uspto.gov/id-master-list-public.html

Under "ID Master List" just enter a category of goods/services (e.g. "Shirts") into the search field, and when you find the relevant category, its class will be designated as the three-digit number under the "Class" column.