I have a use case where i want to invoke my lambda function whenever a object has been pushed in S3 and then push this notification to slack. 
I know this is vague but how can i start doing so ? How can i basically achieve this ? I need to see the structure
You can use S3 Event Notifications to trigger the lambda function.
In bucket's properties, create a new event notification for an event type of s3:ObjectCreated:Put and set the destination to a Lambda function.
Then for the lambda function, write a code either in Python or NodeJS (or whatever you like) and parse the received event and send it to Slack webhook URL.
How to write a AWS lambda function with S3 and Slack integration