Legwork is your chance as a GM to show them the world outside of the mission, and to encourage roleplaying between the party and your NPCs. If you're like me, you have probably figured out lots of interesting bits of backstory that you really want to tell the players about, and this is how you reveal it.
Experienced Shadowrun players will know how to use contacts and resources in the legwork phase, and a smart GM will let them lead. Newer players are often confused by legwork, and it's a good idea to have an NPC fixer tag along and give some tips. I wrote a fairly long post on the subject
here; the rest of that thread is worth browsing as well.
I generally distribute information like so: First, whatever the Johnson thinks they need to know to do the run. Often, this is fairly detailed, since the Johnson wants them to succeed and generally has some solid intel available. This info is presented during the job assignment, and Johnson usually opens the floor to questions at the end. (e.g., "The item is in a lab under this building; The ground floor and above is occupied by an Evo medtech clinic, staffed around the clock by four physicians, six technical staff, seven support staff, and eight security specialists. Due to the upscale clientele, the security is on the Premium Protection plan, with remote Astral and Matrix overwatch at the local Evo hub, estimated heavy response time seven minutes. The ground floor staff are not cleared to know about the basement facility, and we are told that the security in the basement is largely drones and automated systems.")
Once the official info is distributed, we move on to personal knowledge. I like to give information seperately to individual players based on their character's background and knowledges. The ganger might know that the lab is in Hellhounds turf, the ex-corp operative might know that Premium Protection is mandated for low-profile security with around 10k worth of combat upgrades and a focus on social graces, and the guy who used to work for DocWagon might know that the Evo medtech clinics are prime targets for theft due to the chems and tech they keep around, and so they tend to be pretty paranoid and quick to go on lockdown. I would give this info to the players individually, and let them ask questions until they understand as much as their character should, and then have them communicate that info to the rest of the party. This encourages roleplaying and makes the individual characters/players important.
Following that, the players will need to use contacts and resources to get additional information. This is generally when they start to ask stuff that you don't want them to know, e.g. "What Exactly do the Premium Protection units get upgraded with?". It's reasonable that they *could* find the info, with the right approach, so let them explain how they want to look into it, decide if that approach has a chance of success, and have them roll an appropriate dice pool, hidden if possible. I generally give a little info for one hit, solid info for two hits, and really fill in the blanks for three or more. (e.g., "The stock PPU package includes sensory upgrades and a biomonitor, connecting back to a tacnet monitored by an expert system at all times. They also get muscle toner and bone lacing packages. You also find a discussion thread talking about how moving up the ranks seems to require a show of commitment by reinvesting into personal upgrades - most squad leaders and above seem to have invested in some of Evo's more expensive biotech combat upgrades.") This is also a good time to add some misinformation if they roll a glitch, like a rumor that the PPU squads are getting Hellhound handlers because it's becoming "trendy" to have paracritter security.
This is generally where the PCs will take your plot off the rails. They might start to investigate neighboring buildings for alternate entry points, or want dossiers on all three shifts of clinic workers, or try to infiltrate the PPU, or sign up for some elective surgery. It's up to you, of course, but I like the old improv rule of "Never say 'No', say 'Yes, and...'". Try to let the players take the game where they want it to go, and move your obstacles to intercept later. Tunneling in with an Earth Elemental? Great idea! Of course, Evo has Astral overwatch, and now that Earth Elemental is going to need to take on three lesser Fire Elementals that the wagemages have summoned, and the clock is ticking until the Heavy Responders show up...
I hope some of this helps a little, sorry about the length. Good luck with your game!
~Mike