Sarah, first of all let me say, Welcome to VATSIM. It truly can be a great experience. As far as the butterflies of connecting for the first time, you must remember each of us had a 'first time'. By the time I connected to the network for the first time I had already been Simming for 24 years. I knew my aircraft, how to fly it, how to plan a route and navigate it. But boy was live ATC a big difference from the Sim ATC.
Yes, it is unfortunate that not everyone seems to remember what that 'first time' experience was like, but there are many controllers on the network who are always willing to help a new member. That being said, congestion and heavy traffic loads will greatly impact how much assistance one can provide at a given time. So I would suggest starting somewhere that the airport environment would provide you the opportunity to engage ATC with lower traffic levels. In other words, I probably would not start at the major 'hub airports' such as ATL, LAX, JFK, ORD, etc simply because at most times any ATC online at these facilities will have their hands full and there is greater potential that traffic will impact your ability to maneuver for take-off and landing. That is not to say you should never think of flying at these airports I just don't think it is the best place to start.
You mentioned that you hoped ATC would not get mad about you spawning so long as it was mentioned in your flight plan and I am not sure I understand your meaning. ATC will not see your flight plan until you connect and file one so hopefully your meaning is that you will connect first at a ramp area and when ready for departure if your software does not allow for slewing to/from the runway disconnect, relocate and spawn back in with ATC knowledge of what will be happening. Any reasonable controller should be able to work with that scenario and get you headed off on your way.
As for being worried about making mistakes all I can say is don't worry. Because yes, you are likely to make them, and yes, even long standing members with many hours flying and controlling still make them. Heck, if you are listening to Live ATC on a scanner I am sure you hear mistakes made by RW ATC and pilots. No matter how hard we strive for perfection each of us will fail to attain such a lofty goal. Making mistakes is how we learn to correctly accomplish a task, if we take time to heed the lesson.
I look forward to perhaps providing you ATC services in the future and I hope you have a great time here on VATSIM.