If your a fan the Halo trilogy, then Normal should not be very difficult for you. In Normal, enemies do little damage, meaning you can take more. The enemies also have little to no shields, meaning they suck and die fast. Simply play through the game, grabbing other achievements along the way and after some hours, you'll pop this achievement.
The game's to vast to point out specific areas, but if you require help, I'm more than happy to help, simply post a comment or send me a msg here and I'll help to the best of my abilities.
Some advice, grenades are great for clearing enemies (like you didn't know that already ;)), and save points/checkpoints are frequent, you can usually force checkpoints by running backwards and then forward again, so if you get nervous, try forcing checkpoints. Deaths don't matter (unless you have iron on) in story play. Just have fun, and that's easy to do in this game.
Another helpful tid-bit, BR's are very useful, aim for the head (duh) and most enemies can be taken out quickly (Brutes and Grunts mostly). For the Flood, I would recommend the Shotgun.
Use vehicles when possible as they provide extra protection, but if you notice it's on fire, get out of you could die from the explosion.