How to build a simple recommendation system?
How you want to actually store the recommendations is as a question completely unrelated to how one would actually implement a recommendation engine. I leave that to your database architecture. On to the recommending.
You said "simple", so a Pearson correlation coefficient might be the thing you need to read up on.
Calculating such a thing is dead simple. Concept, example code.
The table can be stored in three columns
user_leftuser_topcorrelation
(I have no experience with determining correlation, though)