For starters, and this is a huge one, you're going to need a wideband 02 sensor. Without this, you're "flying in the dark" and might as well take your first run over a mine field, as saying a mine blew your motor up would be a cooler and more respectable story than that of you grenading it yourself. The reason for the WB02 is, you must know what your air to fuel ratios are.
Your stock computer can handle the fuel delivery pre-boost, as this is essentially no different with or without your turbo. However, once you crossover, your computer no longer accurately comprehends the scenario at hand, and you must at this point implement some form of fuel control: enter your FMU.
An FMU is a rising-rate fuel pressure regulator. The more boost that is seen, the greater the fuel delivery. Again, this is where the wideband 02 is a must. The stoichiometric ratio for most gasoline mixtures is 14.7 to 1 (the golden ratio if you will, at which all parts are burned equally). As previously mentioned, pre-boost, your car's computer will do it's best to regulate this ratio, adjusting it accordingly under load (increasing fuel, rich) and upon deceleration (decreasing fuel, lean). But once again, you will physically need to add more fuel when in boost to compensate for the additional turbo-generated air volume, as your car's not going to do it for you (unless you venture down the path of an electronic tune via HPTuners, MegaSquirt, etc.).
If you were to get your turbo in, tune nothing, then drive into the wonderful land of boost, your motor will go "boom". The reason being is you will excessively lean out your AFR to the point of detonation. Now, under boost, and in the extremely general realm of all things considered, you will now want your AFR to read somewhere in the neighborhood of 12.5:1 under load, a little on the rich side of things. Either way, too rich is always better than too lean.
Alrighy then:
1) Get a wideband 02 sensor
2) Tune fuel pressure accordingly with your FMU
Then you can start to mess with timing. But for now, concentrate on fuel.
Also keep in mind that the above was relayed in it's most basic application and theory. There are many, many factors to take into consideration that are worth the life of your motor to read up on, but hopefully this gives you some kind of knowledge base foundation from which to build upon and explore.
