How do I predict a set of frequently bought items?
The simplest way is to build a matrix representing how often two items are bough together: $M[i,j]$ is the count of how many times items $i$ and $j$ are bought together (note that you need to fill only a diagonal matrix, since it's symmetric).

From there you can calculate whatever you need: most frequent $B$ item bought with a given item $A$, top most frequent pairs of items in general, etc.

You could also consider the matrix as a graph representation, and use algorithms specific to graphs.