You probably want the reference-point mode as described here.
You will also need a bed or gtf file with the gene regions and via the --referencePoint parameter you specify that you want the TSS (rather than the TES or the center of the regions in the BED file).
$ computeMatrix reference-point \ # choose the mode
--referencePoint TSS \ # alternatives: TES, center
-b 3000 -a 10000 \ # define the region you are interested in
-R genes.bed \ # the gene annotation of your choice
-S log2ratio_H3K4Me3_chr19.bw \ # your coverage file
-o matrix1_H3K4me3_l2r_TSS.gz \ # to be used with plotHeatmap and plotProfile