What part are you having problems with? You say you want to relate No with DocumentNo. This feels trivial to do as long as those 2 columns ARE actually related.
What I think would help is if you could:
1 - post DDL and sample data so we can see your tables
2 - post what you have tried that didn't work
3 - post your results
4 - post expected results
With what you posted, it sound like you are just trying to do:
FROM [dbo].[Table1] AS T1
JOIN [dbo].[Table2] AS T2 ON T1.No = T2.DocumentNo
BUT in the above, I am making a LOT of assumptions. I am assuming the JOIN type, I am assuming you want all columns, I am assuming you want all rows, I am assuming by "relate 2 tables" you mean you are wanting to SELECT the data, No and DocumentNo ARE actually relatable in their current form (ie don't need to CAST/CONVERT either of them or do any tweaks to them such as truncating any data prior to joining them), etc.
You could very well be talking about making a foreign key to relate the data in which case the SELECT query I gave above makes no sense.
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!