Given a random section of text delimited by line breaks, get the first paragraph
Stating Your Requirements
It's important to have a clear defintion of what you want to achieve before you start writing code, even though there are many different ways of achieving that, for instance by practicing Test-Driven Development or writing a formal specification.
The important part is that without a clear definition, you can't validate whether you're done. In your case, the question contains a description that is completely different than the code and quite unclear.
The above is vital even if you're only writing the code as an exercise for personal use or learning.
Testing and Edge Cases
In the following code:
word_in_next_line = lines[li+1].split()[0]

Why are you assuming that

there will be a next line? What if the text consists of only one paragraph?
the next line will not be empty?

These assumptions are unreasonable and when I first tried out your code on some text, it immediately threw an exception.
Naming

Be careful with historically significant terms such as ss (Google it if you don't know what I mean).

Expressive names are better than abbreviations! Replace:

ss with source_text
ll with line (this looks like the number 11!)
lens with line_lengths
avglen with average_length
maxlen with maximum_length
in the for loop, li with index and ll with line



Conclusion
Without a clear explanation of what you are trying to accomplish, how the input data looks and how you define a paragraph, it's impossible to show you a better way of solving the problem.