Added pod files and pushed. How to undo? how to use gitignore in Xcode & github?

That's correct, you need to add the Pods directory to your .gitignore

1) Remove your files from your github repository:

git rm -r Pods/

and don't forget to commit and push

2) Create a gitignore file:

1. Open terminal and go through your project folder where the .git folder is located
2. Type touch .gitignore
3. Type echo "Pods/" > .gitignore

More informations : here

This is what Cocoapods official documentation has to say.

Whether or not you check in your Pods folder is up to you, as workflows vary from project to project.
It recommends that you keep the Pods directory under source control, and don't add it to your .gitignore. But ultimately this decision is up to you:

Benefits of checking in the Pods directory:

1. After cloning the repo, the project can immediately build and run, even without having CocoaPods installed on the machine. There is no need to run pod install, and no Internet connection is necessary.
2. The Pod artifacts (code/libraries) are always available, even if the source of a Pod (e.g. GitHub) were to go down.
3. The Pod artifacts are guaranteed to be identical to those in the original installation after cloning the repo.

Benefits of ignoring the Pods directory:

1.The source control repo will be smaller and take up less space. As long as the sources (e.g. GitHub) for all Pods are available, CocoaPods is generally able to recreate the same installation. (Technically there is no guarantee that running pod install will fetch and recreate identical artifacts when not using a commit SHA in the Podfile. This is especially true when using zip files in the Podfile.)

2.There won't be any conflicts to deal with when performing source control operations, such as merging branches with different Pod versions.

Whether or not you check in the Pods directory, the Podfile and Podfile.lock should always be kept under version control.

Reference Link:Cocoapods Official Documentation