If you have used FL Studio for more than just starting it up, you might have found out that available instruments are limited. But the good thing that there are loads of VST instruments (free and paid) over the internet. And it's not just instruments, it's also mixer effects!
So let's find some VST right now, go to google.com and search for free VST instruments. You should have plenty of links displayed. I used vstplanet.com VST instruments Just find out any plugin and download it.
When you download any VST plugin, it should come as .dll file which you have to put inside specific folder, which is C:\Program Files\Image-Line\FL Studio 10\Plugins\VST Folder name might be different when using other FL Studio versions.
Once you have placed the VST plugin where it belongs, start up FL Studio. In the top menu, select Channels>Add one>More
Currently you should have new window in front of you, in the bottom of it select Refresh>Fast scan
Now if you've done everything correctly you should have your plugin somewhere in the list, it will be in red color as you have just added it, simply click the checkbox near it and close the window.
That's it! You should be able to add your new instrument to sequencer like any other. Right click on any instrument in sequencer, select Insert>name of your plugin and it's done.
In case you're adding effect/mixer plugins, instead of going to Channels>Add one>More, go to mixer (key F9) and click on the dropdown box in the right side, take Select>More
and do the same steps as for instruments.
There are many free VST plugins for FL studio, and they are easy to find, just google "free VST plugins".