If you have not already done so, download Avogadro for free and watch the tutorials if necessary.
Build an RNA strand on Avogadro (sequence Build → Insert → DNA / RNA) combining the nitrogenous bases as you like but trying to obtain a well identifiable helix even with several attempts.
Help yourself with geometry optimization (sequence Extensions → Optimize geometry) and rotating the molecule (with the help of the DNA tutorial).
Study the properties of the molecule and of the atoms that compose it.