Assuming you get to the University of Sheffield then this is where the social scene is most likely to start, before you gain the confidence of knowing where you are to explore the city.
So here's my guide to the Sheffield Union of Students' Building.
First things first, no building that's on 4 floors is going to be easily navigable, so the map is attached. 
Every Sheffield student knows of this venue, accessible either by going through the side entrance on Glossop Road, or by going through the main doors on the Western Bank side and going down two levels. Cheap cheap drinks every night of the week; 8 pool tables at 50p a time, and loads of TV screens for your sporting events, and coverage of student favourites such as The Simpson's. On a nice day, the outside gardens and sunshine deck make a great place for BBQs and the likes.
Fusion & Foundry
Home of Ent's nights Climax (LGBT Friday Monthly); Population (Mondays weekly); Tuesday Club (Weekly), Roar (Wednesdays weekly), Live Wire (Thursdays weekly) Pop Tarts (Saturdays Weekly, 60s thru to Today). Capacity of the two rooms together, 1200.
Tuesday Club - Drum & Bass in one room, hip hop in the other. Several resident DJs and guests as well. One of the bast DnB nights around.
Roar - Wednesdays. Standard pop/chart stuff in one room and a classic request party in the other. Especially popular with the sports teams. A messy night, very popular. £1 for vodka, sours and sambucca shots.
SPACE - Fridays. Not in Fusion and Foundry but in the Octagon, usually has cocktail offers on and other good drinks offers, one of the cheaper places to go on a Friday as town gets expensive. Plays dance music generally, the sort of stuff that is in the charts.
Pop Tarts - Saturday night retro party. 80s & 90s & 00s music in foundry, 60s & 70s in fusion. Cheap drinks offers & good for a laugh.
Then at special events, eg. Halloween, Freshers Mania, Big Night Out at end of January Exams, Summer Social/Carnival and Mega Roar at Christmas, there are massive multi venue nights where you get a wristband and can gofrom venue to venue as you please, each one hosting a different club night.
Climax (monthly) - LGBT night - fairly relaxed though, plenty of straight people do still go. Has interesting themes sometimes, and occasionally free massages in the 'pink powder room' (fusion)
Also a venue for bands. This year had the long blondes & ash in foundry, and brakes & captain in fusion. Plenty more as part of other events. Radio 1's In New Music We Trust also recently held DJ sets here. (January 2011)
The SU Auditorium
Many lectures happen in here during the day, delivered to first year students due to the size of the groups being taught. But in the evenings the Film Unit have it a couple of nights a week all year, and with a capacity of 400; they host movies delayed by about 8 weeks, at £1.80 for a single ticket, or indeed for a years pass £30.
Interval Cafe Bar
Relaxed and refined, the Interval is the laid back sister of Bar One. Its leading bar design is unique amongst Student Unions as is its service. From hot breakfast sandwiches to pasta, wraps and speciality vegetarian and vegan dishes, the Interval provides food right into the evening. You can accompany your continental cuisine with continental beverages too - the Interval has a selection of international beers and wine.
The main level from the entrance off Western Bank.
Does what it says on the tin, a vital place for your ents tickets, also issues tickets for Sheffield International Venues & Theatres (Hallam FM Arena, EIS Sheffield, Crucible, Sheffield City Hall, Lycyem, IceSheffield and Ponds Forge ISC).
Student Advice Centre / Sheffield Students Information Centre
SSiD - Information point, for all your forms, detail changes, and in the event of any general local information needed. Advice Centre - Free, professional advice service on everything from employment, housing, money issues, student visas etc.
Sheffield University Branch
All your Sheffield University hoodies, T-Shirts, Sweaters, and goodies.
Student essentials including stationary, magazines, and food/drink
Fairtrade coffee available here, quite expensive though.
Custom make your own salads. Offers a selction of freshly made smoothies and jacket spuds.
The Activities and Sports Zone
Local information point. Lower floor of the 24 Hour Revision Centre during Exam Periods. Also houses Give It a Go, the termly programme of one-off activities to try things out.
The Copy Shop
They'll do anything if you give them the artwork, large banners, posters, business cards...
Mainly used by 2nd / 3rd years bringing their stuff down, as all 1st year accommodation has some form of laundering facilities on site.
Some Sabbatical Officers' Offices on this level also. More Vending Machines
The upper level of The 24 hr Revision Centre during Exam Periods, has more Network enabled computers, coffee / soft drink machines, and plenty of desks as well as comfy couches.
More Sabbatical Officers' Offices
Loxley Food Court
Sally's Coffee Lounge University owned food venues, but very nice if you want to sit down, read a news paper and eat/drink for a bit.