The RaspberryPi 3B+ has a full gig of ram, but while running MongoDB (and also when building from source if you choose to do so) this may not be enough. So before going any further you will first want to create some additional memory capacity by adding a swap file to your filesystem. Run the following command to create a new 2GB swap file, start swapping on it, and automatically remount the swapfile on reboot:
sudo touch /tmpvar/theswapswap
sudo chmod 600 600 /tmpvar/theswapswap
sudo dd if=/dev/zero of=/tmpvar/theswap swap bs=1M count=2048
sudo mkswap mkswap /tmpvar/theswap swap
sudo swapon /tmp/theswapswapon /var/swap
sudo echo "/var/swap swap swap defaults 0 0 >> /etc/fstab"
You may also want to be able to connect and manage your device remotely, in which case you should also install: