Weapons – There are a lot of nasty folk out there, best to always have a weapon to hand just in case, never know when someone might jump out at you!
Walk the grid! - If you want to get all of the achievements such as finding all the birds/metal pieces and using all the weapons then you need to be pretty thorough and explore all areas. Like any forensic investigator you'll want to check everywhere around you including up high and down low. Remember the crime scenes are 3D - Walls, floors and roofs. Note that you can also move some objects.
Use the Taser - The taser is a pretty great way of disarming the bad guys so use it to your advantage to steal their weapons or to get in a few good hits before they recover. For some enemies the taser will be enough to finish them off.
Use the torch - Seems like common sense but for most of the game the torch is going to always be on. Not sure if switching it off will stop enemies from spotting you but either way it is a key piece of kit so use it.
Forensic tools - Most of the forensic tools are easy to use, you are shown how to use each one to begin with so just remember how they work and what each one is designed to identify or capture. At the bottom right a message will appear telling you to press X whenever you need to use your forensic tools.
Block – With all melee weapons you can block attacks from the enemy, it is very useful as it stops them taking away your life and also allows you to quickly counter attack. You need to get your timing right but this shouldn't be too hard to get used to.
Birds & Metal Pieces – If you are looking to collect all of these then your going to have to be quite thorough when searching levels. Most metal pieces are found near walls with eyes on them so that should help a bit. As for the birds listen out for the alive ones – you should be able to hear them. Also some of the others have flies around them which you can also hear when you are close.