Hi & welcome to the forums.
First step for any new user should be to check out the free user guide, which you seem to have done, AND check out the free video tutorials. Kenny Gioia`s in particular are excellent for helping you get your head around how DAWs in general and reaper in particular work.
Understanding the concept of HOW plugins work and more importantly HOW they should be installed & accessed is not that complicated but there are pitfalls. Kenny covers them all really well.
A couple of questions that may lead you in the right direction.
Where did you install your plugins?
One of the classic newb mistakes is to put third party plugins in the Reaper plugins directory, despite there being a warning message in that file that it is NOT intended for anything but reapers own native plugins, which are not vst or Au or any of the other formats.
Best idea is to let the plugins decide for themselves where they want to install, but if they give you the option, create folder(s) to take them. Typically, vst and vsti in a folder like C:\Program Files\vst plugins64 or \Program files (x86)\vst plugins32 for 32bit one.
Enter these in the vst locations window which you will find at options\preferences\plugins\vst\ and then do a clear/rescan, which will tell reaper where your plugins are.
When you then try to add a plugin to a track, the plugin listing window will open and show you the plugins you have installed. Whatever you do, DO NOT just add C:\ or C:\ program files to your location list, as this will then search (for ages) for every .dll file on your computer. There are a gazillion of them and almost none are audio plugins!!
Let us know how you get on.
Bugger! Softsynth started typing long after me, but I talk too much!