I highly recommend beginning on the easiest difficulty. It's too easy to get overwhelmed by enemies in higher difficulties and also the game throws in red herrings such as names of people who are not involved in the crimes. 

I'll tell you right now, learn to crouch and hide behind things. Also learn to jump over furniture as it will occasionally block you out of places unless you do. Finally, remember that the point is to gather clues and arrest people. Killing enemies is not the objective and there's not much reason to use lethal force at all unless you can do it without triggering an alarm and steal the enemy's clothes. Otherwise, it wastes time, attracts more enemies, and distracts you when you could be snooping through files and picking up names or places to look for.

Also, keep a pencil and paper by your pc. Write down names of people, places and crime organizations as they come up. While in stealth sequences, you can run a search on a person or place using an enemy terminal and possibly find more info on the person and their involvement. However, you need to write it all down since your crime data cannot be accessed during break-ins. Also, during a break-in write down any new stuff not already in your notes. (Example: I had all participants names and location except the mastermind's. During a subsequent break-in, I photographed a file that mentioned Istanbul. None of the other participants were in Istanbul. I guessed correctly that the mastermind was hiding in Istanbul. I did a search on Istanbul during the same break-in and found the address for the Stassi in Istanbul. Flew to Istanbul, broke into the Stassi hideout, found incriminating evidence against the mastermind, found the mastermind,  and arrested them.)

Computer Functions:

Enter person's name=Get photo, learn org they work for, city they are in, etc

Enter city name=Get address of a crime org branch location in that city, the same crime org you are currently breaking into (Example: You're breaking in PFO office in Washington, search for Istanbul, computer will give you address of PFO cell located in Istanbul)

Enter organization name=Possibly get address of that crime organization's location in the city you are currently in. (Example: breaking into MI6 offices in London, search for Stassi, computer will give you local address for Stassi cell.

I know enough about the game to be able to give answers to a lot of questions. So ask away.