Madhan GST is automatically calculated in Tally if you have configured GST correctly.
Here is a link to configuring GST Taxation Ledgers in Tally in a correct way - https://www.tallyschool.com/gst-taxation-ledgers-in-tally/
For services, you need to create ledgers as Sales under Sales Accounts and for Type of Supply should be Services.
For example, if you are selling architecture services, it should be created as Architecture Services under Sales Accounts with GST configured as Services in that ledger.
Finally, you can pass sales or purchase voucher for that but it will be Accounting Invoice and not Item Invoice which we generally do when we are selling products.
But, when we sell services, we don't sell any products, hence no stock items are required and therefore no item invoice.
I hope you got it. If you still have any questions, please feel free to reply to this post.