It depends on the tank for both. If you had a 10 gallon aquarium and didn't mind buying RODI water, you might not need to spend the money on either. However, if you have a larger tank or don't want to head to the LFS for water, a RODI is a must. You can buy some inexpensive models to start.
A skimmer is a great tool, but I would put it as less of a necessity. A lot of reefers run them, however you can get away without one if you're diligent about your water changes, have a refugium or algae turf scruber, etc. I would start with the RODI then determine whether you want/need a skimmer based on your tanks needs. For example, if you like softies and keep a smaller bioload, then a skimmer certaintly isn't going to be necessary (but may still help).