How to start this problem? Dihybrid cross?

Unfortunately this question doesn't really make sense. In your first paragraph, when you talk about the "next cross," are those the plants that you already have and are planning to use as the parents? Or are they the intended offspring? Also, what do you mean by "with white edged with red flowers"? Is R the gene for "red" and little r the gene for white edges? This is not indicated.

I will try to do the problem, assuming that the two bushes in paragraph 1 are the parents and you are trying to find the offspring, with r as the gene for white edges.

A homozygous short rose bush (tt) with a red flower (RR or Rr) has a genotype of either RRtt or Rrtt.
A heterozygous tall rose bush (Tt) with white edges (rr?) would have a genotype of rrTt.

So the offspring from crossing RRtt x rrTt would have a 50/50 ratio of RrTt to Rrtt (in other words, all offspring would be red, but 50% would be tall and 50% would be short)
A Rrtt x rrTt cross would produce genotypes of RrTt, rrTt, Rrtt, and rrtt (so you would have red, white-edged, tall and short offspring produced)