I am a new eOS user and I can't find a particular application in the store. Do I need to do anything in particular before trying to install directly from .deb file?
You can give Eddy a try if you prefer a nice graphical interface.
If you're comfortable with the command line, you can use the
dpkg command to install a .deb package.
The format would be
sudo dpkg -i name_of_package.deb
sudo apt-get install -f. the
dpkg -i command may leaves the package status in an "unconfigured" state due to missing dependencies.
sudo apt-get install -f will try to fix it downloading and installing those dependencies
If you're looking for a graphical install, you can use the Software Center or the Gdebi application.
This website might help you: https://itsfoss.com/install-deb-files-ubuntu/