First you need to download and install the virtual instrument that you want to use. That is done separately to Reaper so that it can be used with any DAW.
I use MT PowerDrumkit, which is free and have installed it inside the following directory: C:\Program Files (x86)\VST (although it's up to you where you put it).
Next you tell Reaper where to look for it.
Use the Options > Preferences menu, then scroll down to Plugins > VST and click on that. At the top of the right hand side you will see a setting called VST Plug in paths.
You need to add the directory where you installed your VST to the end of the list (if it's not already there).
In my case it had the following there:
C:\Program Files (x86)\Steinberg\VstPlugins;C:\Program Files (x86)\VST;C:\Program Files\Common Files\VST3
then I put a semi-colon and then the folder name to make it look like:
C:\Program Files (x86)\Steinberg\VstPlugins;C:\Program Files (x86)\VST;C:\Program Files\Common Files\VST3;C:\Program Files (x86)\VstPlugins
Press on the Apply and Ok buttons at the bottom of the panel. You will need to close and re-open Reaper before it will check for new virtual instruments
To use the instrument in a song create a new midi track.
Then click on the FX button which shows you the current list of FX applied to the track. Its probably empty right now. You need to add a new one so click on the Add button (bottom LHS). This will open up a search panel which already comes with a lot of pre-loaded stuff from Reaper so it's a bit cluttered. You can use the 'Filter' field to search by name, or you can click on the VST, VSTi, VST3, VST3i categories on the LHS. But since this requires you to know what category your instrument is, it's usually easier to search by name or even just scroll through the list until you find it. Double click on that to add it to the track.
Your virtual instrument should now be ready to use. You can record to it from an external midi keyboard, by drawing notes using the midi editor, or using the virtual keyboard in Reaper.
As an aside, if you happen to use MT PowerDrumkit, you will get a message when you open it asking you to donate at their website. You are not required to, but you do need to press the SKIP button if you dont want to - otherwise the drumkit wont work. And have you have to do this each time you open it, or open an existing song that uses it. That only applies to MT PowerDrumkit, but watch out for different messages from other VSTs. If it is not working, make sure there isn't a message screen hiding somewhere in the background that you have to click on first.