It looks as if you have a particular set of transmitter and receiver modules. I think in order to better answer your question, it would be useful to know what modules you have.
In general, you can use a wire for an antenna. It is best to have 1/4 wave and even better if you attach an additional 1/4 wave to the ground side of the output.
https://www.easycalculation.com/physics/electromagnetism/antenna-wavelength.phpI can't say for sure without looking at the data sheet for the RF modules but I'd guess that the DO pin is data. It's best to look at the data sheets to be sure.
