A
BIG BIG part of vatsim is using charts. Check out
www.myairplane.com. Next time you fly around OFFLINE, practice using charts, step by step.
For instance, print out the taxi-charts for the airport you're landing/leaving from, and spend all the time you need to to navigate step by step according to what the computer atc is telling you to do. Don't feel discouraged, this can take PLENTY of time to learn to read. I still get lost at big airports sometimes. Ask ATC if you get stuck to give you directions like "left" or "right." If you just need clarification.
Next get some approach training. If you look at all the STAR (standard arrivals) approach charts for an airport, you'll get some idea of how they work. It'll take a lot of paper, but a nice way to visualize it is print out all of the STARs for an airport and lay them out on the floor. For instance, if you're flying into KORD, you'll see 5 approach plates. Put them on the floor and match up all of the little "ORD" markers (since all STARs head towards that marker). You'll notice for instance that Bradford comes at chicago from the SW, Janesville from the NW, Knox from the SE, etc.
So think of it this way, if you're flying into chicago from toronto, you'll be coming at it from the North East, And according to all the STARs you're looking at, you'll be coming in on the Pullman 4 approach. In otherwords, By the time you reach chicago's airspace from your flight, you should be coming in through one of these controlled corridors to keep from running into other airplanes.
If you see on the top of the chart, there are 3 possible places to intercept this path, each having their own lines/set of waypoints on their way to the main path that starts at PMM (Pullman). Those little triangles are fixes that you can use to navigate along the path. Written along these are the altitudes that you want to be at as you're approaching. Note that they're different for each one. When you're flying with ATC coverage on VATSIM, they'll tell you these altitudes...usually. For instance, you could hear "Cross PMM at 10,000." This means that wherever you are, start descending until you hit 10,000 feet, and then after you cross that waypoint, they'll give you a new altitude to descend to. NOTE: Not all ATC give you these, so always keep in the back of your mind about when you want to descend, and if ATC hasn't given you a new altitude ASK! They're more than willing to assist. Plus, just like us, they make mistakes and lose the occasional plane too!
The final part of the approach involves heading directly to the ORD waypoint, and wait for ATC to vector you to your runway. You can see more detailed approaches from the ORD to the different runways themselves, but ATC will always give you those vectors.
For departure it's much easier, just let ATC vector you to one of the waipoints you see on "Ohare 1 departure" (top of the page at myairplane.com list) and follow his instructions from there. NOTE: One thing most of the controllers I've flown with DON'T say "resume own navigation" like you're used to hearing from the default flight sim. Keep your eyes open to when you cross that departure point (which should be listed in your flight plan), and once you're passed there you're on your own.
As far as the lingo, go through the pilot training program on the VATSIM website. I usually keep the window that explains all the necessary phrases open so that I can just hit Alt+Tab in case I get lost (which is easy the first few flights).

Looks like I typed a lot for you! If you're still reading by now

, let me give you the most important advice about flying on VATSIM:
It's okay to sound like an idiot your first few times and get totally lost, EVERYONE does.
I've flown plenty of times with my virtual airline, and even with them coaching me through I've messed up my fair share of phrases, transponder codes and when to actually turn them on to active, course adjustments/altitude changes, speed restrictions, etc. I've seen more go-arounds (especially on tricky approaches) than you would guess, and I've still come out alive. Sure you get a little red in the face a lot of times, but soon it'll be fun, and one of the most realistic experiences you'll have!
P.S. A handy little piece of software to use with SquawkBox is "ServInfo." It's a simple program that you can just run and get a list of all aircraft on the server, along with either a list or a map of all controllers on line. It helps for your first few times to find an area where there is ATC coverage, but not a lot of planes.
See you in the skies, look for my callsign on the server (UAL2203) and drop me a private message (eg: type ".msg UAL2203" and then your message).