Punch Radroaches. Low-level Radroaches pose little threat, but they can nip you to shreds early in the game. The easiest way to kill them is to ready your fists, or any melee weapon, and give 'em a quick whack. Don't even bother wasting your Action Points by entering the VATS targeting, just squash 'em.
Shoot dogs, moles, and basic critters anywhere. These low-level animals aren't much of a threat and should go down quickly with a few shots to the head or torso. If you get swarmed by a pack of dogs, consider lobbing a grenade or two.
Shoot humans and Super Mutants in the head. The most obvious weak point is also one of the most common. If you're fighting a humanoid enemy, it's a safe bet that the head is the highest-damage target.
Consider investing in the Animal Lover perk. The mutated creatures that roam the Capital Wastelands make up some of the more difficult enemies in the game. To save yourself some trouble, you might want to invest in the Animal Lover perk — it will prevent animals from attacking you on sight. Subsequent levels will make animals fight for you.
Shoot Ants in their antennae. The oversized Fire Ants that infest Greywater make up one of the most frustrating battles in Fallout 3. The secret is to target and cripple the ant's antennae, and then let them burn each other to a crisp in the confusion.
Shoot Centaurs in the legs. These abominations serve as watchdogs for the Super Mutants. Aside from their radioactive projectiles, Centaurs don't represent a big threat but can absorb some impressive damage. To take 'em out quickly, target their legs.
Shoot Radscorpions in the stinger. Radscorpions are one of the toughest creatures in Fallout 3, and can absorb an impressive amount of damage. Keep your distance and use your VATS targeting to shred their stinger gland. Still, be prepared for a fight.
Shoot Mirelurks in the face. These territorial crab-men can overwhelm and destroy you with their brutal pinchers. The trick is to use a powerful weapon — the combat shotgun works well — to repeatedly target their tiny, exposed faces. You'll need to get up-close-and-personal to hit your mark, but nobody said it would be easy.
Use Pulse grenades and Pulse mines against robots. Some of the toughest enemies in Fallout 3 are robotic in nature. In these cases, Pulse grenades and Pulse mines are the weapon of choice — robots can't survive the electromagnetic blast.
Shoot Protectrons in the chest. These low-level robots are easy to defeat, if you can get past their laser fire. Simply pump a few high-powered slugs, or a few close-range shotgun blasts, into the Protectron's torso to drop it for good.
Shoot Robobrains in the brain (duh). These slow-moving robots can inflict big damage, but they also have an obvious weak point: a glass-encased brain that rests on top of their armored chassis. Give 'em a few hard hits to the brainpan and they'll go down in a jiffy.
Shoot Mr. Gutsy in the sensor array. These high-level robots are very tough. Your best bet is a Pulse grenade or mine, but those are hard to come by. In a pinch, you can use the VATS targeting to take out Mr. Gutsy's sensor array, effectively blinding it. Then you'll have more leeway to finish it off with your weapon of choice.
Save cash by drinking water. Stimpacks are awfully pricey, starting at 35 bottlecaps. The budget solution is to use water sources to heal yourself whenever you're traveling the wastelands. Though you'll gain radiation from drinking, it's cheaper to use a Rad Away or two over time than several dozen Stimpacks. Fire hydrants, sinks, and drinking fountains harbor the lowest rad levels, so drink up!
Beef up your Medicine skill. For all their expense, Stimpacks don't heal very much unless you increase your Medicine skill. Shoot to hit at least 50 in your Medicine skill, or you'll be guzzling those pricey Stimpacks every few seconds.
Heal crippled limbs first. It's easy to overlook the condition of your limbs when you're struggling to stay alive in a battle. But if one of your limbs becomes crippled, you'll take double damage from any subsequent shots to that limb. Crippled arms will also severely impact your weapon accuracy, so it pays to keep your limbs patched up as much as possible — one button press (Square on the PS3, X on the Xbox 360) will allow you to focus a Stimpack on any limb you choose.
Sleeping will heal your limbs. If your limbs are in tatters, a quick hour-long nap will be enough to fully regenerate them. Ignore the health of your limbs at your own peril.
Certain foods heal more than others. Much of the food you'll scavenge is of low nutritional value and won't heal more than a measly 5 or 10 hit points. But several foods — most notably Mirelurk meat and Mirelurk softshell meat — will heal far more. Pick your food choices carefully.
Corpses are part of a balanced breakfast. By selecting the Cannibal perk, you can restore lost life by eating dead bodies — handy, as you'll be seeing plenty of them. Indulging in your zombie tendencies will deplete your Karma and turn any nearby humans against you, so be warned.
Buffout boosts your Strength and Endurance temporarily. Buffout is especially handy if you want to carry extra equipment for a short period of time. If you want to carry an oversized load of gear back to town for re-sale, pop a Buffout, load up, and quick travel back to town.
Mentats increase your Charisma and Intelligence temporarily. Mentats are the least useful of the chems in Fallout 3, but they're useful if you need a quick boost to your people skills or your hacking abilities.
Psycho will boost your damage by 25% temporarily. Psycho is best saved for difficult battles, when you need an edge in combat. They're rare, so make the most of them.
Jet will replenish some of your Action Points. When your Action Points deplete, you'll be unable to use the VATS targeting system. If you can't afford to wait for the Action Points to regenerate, popping some Jet will keep you in the fight.
Alcohol will give you beer muscles. Like a low-rent version of Buffout, alcoholic beverages will temporarily boost your strength. This is handy if you have a big load of salvaged gear you need to take back to town for re-sale.
Don't get addicted! With repeated use, you can become addicted to any of the chems in Fallout 3. Some of the addictive side effects are worse than others, but if you need to detox you can always pay your town clinic to kill your addiction.