When an Allele is dominant that means that it will show in a phenotype over the recessive Allele, if present, in the genotype. Since the Allele for tall plants is dominant, it will show in the phenotype, even if a short allele is present. Thus a genotype of Tt will show tall plants, because the dominant "tall-plant" allele is present.
If you had Genotype TT, you would also have tall plants. If you had genotype tt, you would have short plants, since the dominant Tall-plant allele isn't present.