If you've never played (the original) UFO/TFTD, I recommend you reload save games when you see a catastrophe happen. If you don't, you quickly run out of money, soldiers and equipment, and it's a situation you'll never recover from anyway, even if the "Game Over" screen is still months away.
In any case, in OpenXcom you're expected to choose your battles : It's tempting to try attack everything to maximize income, material and combat experience, but you should always remember that no battle is mandatory, and even mid-battle, retreat is possible.