I am getting an RCX and I know it is ether a 1.0 or a 1.5. Which program do I use to program my RCX?
You really have two major choices:
Try to acquire, install, and run the official Lego software for the RCX.
Use some third-party tool to develop programs that can be uploaded and run on the brick.
(1) is getting harder and harder to do on modern computers. Personally, I would not bother trying.
(2) means choosing some third-party tooling. The other answer that refers to BricxCC falls into this category. (You should check out the "RCX" section at that link for more information.) Many of the third-party tools for the RCX brick are pretty much no longer under active development.
But this does not mean that they don't work, or aren't useful.
Some decisions you need to make are: do you need an IDE? Are you comfortable at the command line? What platform will you be hosting the development tools on? Do you want to use the default firmware, or use a third-party that replaces the default firmware?
For example, there is a tool called NQC which is a C-like language and brick API with a facility for uploading compiled brick programs and firmwares to all RCX bricks. It can also set the clock, launch specific programs stored on the brick, and even use the brick as a datalogger. It requires an RCX 1.0, 1.5 or 2.x compatible firmware.
LeJOS is a tiny Java compiler and runtime that needs its own firmware which it loads onto the brick. I think people have figured out how to use Eclipse as an IDE for LeJOS.
Full disclosure: I maintain my own fork for NQC on GitHub, and I'm pretty sure it is the only fork under semi-active development. If you need official firmwares for the RCX brick, I've included them in this project as a convenience. Remember that you can use any compatible firmware on a 1.0 or 1.5 brick. That is, if you want a 2.x brick, just install the 2.x firmware.
I like http://bricxcc.sourceforge.net the best for the RCX! Keep in min that the RCX will loose it's firmware if your batteries are empty or - when powered via 9V - it loses it's power.