See here for some good guidelines:
github.com/ozbillwang/terraform-be...

In general you need to think hard on the structure of Terraform, I propose you do these 3 things;

• use modules, keep the general resources code separate from your solution code. Later on you can even put the modules in separate git repo's and use versioning to avoid changes from breaking other solutions.
• use a solutions/services directory, this is where you wire up all the modules together into a solution or a service. Basically your application.
• use an environment directory, this is where you define the exact parameters of your deployment per environment (dev, test, production). There are differences and you should keep them separated at all times.

This could look like this:

- modules
  - ec2
  - vpc
  - rds
  - apigateway
- solution
  - api
  - application_a
- environment
  - dev
  - test
  - prod

You can check out my public repo for an example:

I highly recommend getting some structured training to become familiar with the fundamentals first. Hands-on courses like Using Terraform to Manage Applications and Infrastructure are likely to answer many of your questions.

There is a lot of free content on these topics around, but IMO paid services like Linux Academy and Cloud Guru provide a higher quality learning experience even if you're not working towards a certificaiton.

Good luck!

Is it good to use terraform for invoking Lambda or serverless is the better solution?

Terraform should deal with declaring and provisioning your required infrastructure only. You can invoke Lambda functions in terraform to fetch data from that lambda function to use in your terraform modules however but it shouldn't be done to perform actions that may change infrastructure or data unknowingly.

If you need to call your lambda functions for another reason it might be good to use something like Ansible to script this action and call this as a separate step in your pipeline.

1. What could be the best practice to set up terraform structure? For me, structure will give others who see your code a better understanding of the way you thinking when trying to solve the problem. So if you shared this project with your team/others, clearer is better.

One of my favorite of the terraform Structure is from Charity. She create the article here: charity.wtf/2016/03/30/terraform-v...

the way how she explains the problem, the process, the way how she looks at that article gives me some idea of how I create Terraform projects.

1. Dividing modules into specific submodules is recommended?
   Depends on how big your projects and how granular you want. If you only work for small services/small projects maybe not too much submodule will be better. Since too much modules/submodules will be more hard maintaining it later

2. Is it good to use terraform for invoking Lambda or serverless is the better solution?
   Depends again. If your terraform is part of big projects for maintaining your whole environments, I think it a bit of risk if you want to use terraform for invoke Lambda. Since one error/problem will risk your state.