Heya JH,
You need an audio interface that lets plug your guitar into your computer. Typically it's a a box box that on one end has a couple of quarter inch guitar jacks, and on the other end has a USB connection. You plug the guitar into the one of the jacks, plug the USB cable into your computer. Here is an example of one of these interfaces that a lot of people seem to like, you can get cheaper ones, but this one has a lot of fans.
https://www.amazon.com/dp/B005OZE9SA
Once you've got this thing plugged into your computer, you should be able to configure Reaper to use it as an audio input. Then it's just a matter of making a new track, setting the input of that track to be whatever port your guitar is plugged into, and hitting record and rocking out.
You'll want to take a look at a visualization of the waveform that you get from recording your guitar and make sure you're getting reasonably sized set of waves, and you're not clipping all over the place. You'll want to adjust the gain, or your guitar's volume knob until you get a sound that doesn't clip or distort.
Now that sound you get out of your guitar is going to sound pretty boring. You'll want to download a free amp simulator to get some better sounds out of your guitar. You can get some free simulated amps from products like IK Multimedia's Amplitube 4, when you download their "custom shop" application. This will be a VST plugin that you can load onto the track that you recorded your guitar on. Other companies that sell virtual guitar amps/effects I think also have a free version that you can try out.
Good luck!