During the Zwicky Transient Facility (ZTF) Phase I operations, 78 hydrogen-poor superluminous supernovae (SLSNe-I) were discovered in less than 3yr, constituting the largest sample from a single survey. This paper (Paper I) presents the data, including the optical/UV light curves and classification spectra, while Paper II in this series will focus on the detailed analysis of the light curves and modeling. Our photometry is primarily taken by ZTF in the g, r, and i bands, and with additional data from other ground-based facilities and Swift. The events of our sample cover a redshift range of z=0.06-0.67, with a median and 1{sigma} error (16% and 84% percentiles) of z_med_=0.265_-0.135_^+0.143^. The peak luminosity covers -22.8mag<=M_g,peak_<=-19.8mag, with a median value of -21.48_-0.61_^+1.13^mag. The light curves evolve slowly with a mean rest- frame rise time of t_rise_=41.9+/-17.8days. The luminosity and timescale distributions suggest that low-luminosity SLSNe-I with a peak luminosity ~-20mag or extremely fast-rising events (<10days) exist, but are rare. We confirm previous findings that slowly rising SLSNe-I also tend to fade slowly. The rest-frame color and temperature evolution show large scatters, suggesting that the SLSN-I population may have diverse spectral energy distributions. The peak rest-frame color shows a moderate correlation with the peak absolute magnitude, i.e., brighter SLSNe-I tend to have bluer colors. With optical and UV photometry, we construct the bolometric luminosity and derive a bolometric correction relation that is generally applicable for converting g, r-band photometry to the bolometric luminosity for SLSNe-I.