Mission: Find the documents in the underground lab[]
The next primary mission is to get into a lab designated X18. When you accept the task the Barkeep will give you a Stalker Suit which will be a major improvement over the Merc Suit you've probably been wearing through Garbage and Agroprom Research Institute. Besides the better stats, the Stalker Suit also comes with nightvision which is a very nice bonus.
To get into the lab you'll need two keys, one of which the Barkeep will already have provided. You'll need to obtain the second one from someone called Borov in the Dark Valley... and by "obtain" the Barkeep actually mean you must pry it from his cold, dead hands.
This may be a good time to buy a new weapon from the Barkeep, namely the IL 86 assault rifle. Although, compared to the Akm 74/2, it has less accuracy, handling and rate of fire. It comes with an integrated scope which obviously is a nice feature, especially at this point in the game. The IL 86 uses a different ammunition type, 5.56x45mm rounds, which can be bought from the Barkeep - in the upcoming areas (Dark Valley, especially) you won't find this type of ammo, so if you decide to pick up the IL be sure to buy as much ammunition as possible. You will get your hands on a PSO-1 scope that will fit on your Akm 74/2 so if don't wish to part with that just yet, you can just bring that along as your primary weapon, or backup in case you run low on ammo for your other primary weapons.
Travel light because the Dark Valley, like many of the other areas, is likely to turn up alot of loot including artifacts and possibly a unique item or two.
Now you have different options available to you - you can simply proceed directly to the Dark Valley if you wish, or you can pick up some optional missions from the Bar area and explore some different areas. If you still haven't picked up the PSO-1 Scope you can actually do so in three places now that you've reached the Bar - you'll still be able to get it in the Dark Valley; you can also pick up a Vodka bottle and head into the Army Warehouses area and locate Screw at the Freedom HQ - he'll give you a scope in exhange for the vodka. Finally you can head into the Wild Territory - this is a tougher place, though, since you'll need to take the scope from four Merc snipers.
As you may have noticed with the inventory of Sidorovich, it is actually randomly generated (more or less) and will replenish when you leave the area and come back later; the same goes for other vendors and can be used to eg. buy ammunititon that you're low on - simply buy as much of the ammo as you can, leave the area and come back, and the vendor should now have more of that particular ammunition available.
The next section is about the arena - a place where you can fight different opponents and win money. If you don't have any interest in that just skip ahead to the next page.