KVR Audio is the place you want. Lots of free VSTs in there. Almost none of them will sound as good as ones you have to pay for though so I'd highly suggest investing in some VSTs sometime in the future.
Basically, what you have to do to install a VST is to first make a folder called VstPlugins (I'd suggest putting this in your Program Files folder). Then, in FL Studio, go to Options > File Settings and under VST plugins extra search folder browse for the VstPlugins folder you made (this may have been done already for you). Now FL Studio knows where to look for new VSTs.
From then on it depends on what VST you're installing. A lot of the better ones come with a setup file (.exe) which should guide you through everything (when the setup wizard asks you where your VST Plugins folder is, obviously make sure you browse for the VstPlugins folder you set in FL Studio's File Settings). For the ones that don't come with a setup file, all you need to do is simply copy and paste the file(s) into your VstPlugins folder.
Once you've gone through the setup or pasted your files into the VstPlugins folder, go to FL again, go to Channels > Add one > More..., and at the bottom right of the window that just popped up, click Refresh > Fast scan (recommended). This will scan in the VstPlugins folder you set in the File Settings for any new VSTs and should only take a few seconds. Your new VSTs will come up in temporary red text in the list of VSTs when it's done scanning. Click on the box near them to favourite them. An 'F' should come up next to them.
After that, simply close that window, go to Channels > Add one and choose your VST. It'll throw it in the step sequencer as a channel and you can start using it. If you still have problems with it, or have any FL Studio questions in the future, just go here.