The first thing you must do is talk to Sidorovich - he'll give you two options: either to treat you like a rookie or like a "real" stalker. The first option triggers a short tutorial regarding your PDA which you'll be using quite frequently. If you are new to the game the tutorial gives you a nice run-through on how the PDA works and what kind of information it provides. If you have played the game before, or just don't really care about the PDA right now, you can ask Sidorovich to just give you a mission; you can study the PDA at any time and most of the different pages and options are easy to figure out anyway.