Hi @VikramM1 ,

You can create a relationship between two tables by creating a reference field on your custom table and point it to another table this is a basic relationship which allow the relation between two tables.

Please Follow Example -

I have created two Tables - Table A and Table B

Table A-

Table B -

Then I have create one field on Table A (Reference to B ) that is referencing to table B

Once this is done we need to create relationship between these two tables 

Navigate to System Definition -> Relationship 

In Relationship we need create relationship between these two tables 

to know more how to create relationship follow link -https://www.servicenowelite.com/blog/2014/2/6/show-attachments-related-lists

Once this is done we need to add related list to table A 

Right click on Table A form -> Configure -> Related List 

Add Related list to table A that we have created 

Once that is done you are able to see related list at the bottom of Table A and you will able to create new record for table B using that related list 

Thanks,

Astik