Tell your Group Lead(GL) before take off, generally through the text buffer as VOX tends to clutter a bit just before frame start, that you're new to the scenario world. He'll likely go over the day's orders, the loadout for the first sortie, and a general overview of the rules (how many lives you get, ROE, where you can rearm or replane, etc.). If you plan to/feel the need to/have the idea to do ANYTHING outside the scope of the orders he's given, ask first. It may be a great idea, it may be against the rules. A good GL would much rather be asked a lot of simple questions than discover one of his group has broken a rule under his nose. Eventually (if you stay in Scenarios) you'll get used to the basic flow of things, you'll get used to picking up on specific information as you read the rules, and you won't have to ask so much...or even become a GL yourself.
Please, please follow the orders you are given. If you discover that it just isn't your gig, don't cause havoc. We've had quite a few of the younger players show up to the summer scenarios and get bored after expecting an immediate dogfight; that's all well and good, their right, etc. Unfortunately, said players sometimes go "rogue" and meander about breaking rules and ruining it for the other players, which then ties up the CMs with the process of removing them from the event...which generally leaves said player with a "they're a bunch of meanies" mindset.
If you discover you don't enjoy the event, we'd appreciate leaving quietly...but, I think you'll like things just fine.
So, in summary; follow the group lead, read and follow the rules, don't cause undue havoc, and you'll likely have a pleasant experience. I find that Scenarios present a unique atmosphere in which even if your group has a bad frame (very few kills, get romped by the enemy), you still have fun. Might rabble amongst each other about how things went, but overall, I find myself back each Saturday.
There are two methods for joining us each Saturday; as a walkon or as a registered player.
Being a walkon means a bit more freedom; you aren't "tied" to the event in any way, but as a trade-off you may not get to fly the plane you want or even on the country you prefer. Once you do fly with a group, however, you generally become sort of a "welcome guest;" most walkons tend to stick with the group they've flown with before throughout the event. You'll note before the frame, the CMs will be calling "all walkons to X field on X country for assignment." This isn't a mandatory process; if you aren't registered, but you know where you want to be and who or what you want to fly, go there first and you'll likely get a spot. If the group is full, though, you may have to be shifted elsewhere.
A registered player signs up for the event beforehand via the AHEvents website. Doing so gives them a guaranteed spot for what they register in, however it also implies a willingness to show up for the entirety of the event (or, if you need to miss a frame or two, specify such in the Comments section of the registration and make sure your GL knows). Because of their guaranteed affiliation, registered players gain access to a special forum for the side they signed up for where orders, strategies, and other country-private things are discussed. Most importantly, this is where the final orders are given out. If you have your orders memorized before the frame, it's much easier on the GL. If you're a "command staff" member (CO or XO for the country, Group Leaders, and sometimes Flight Leaders), you'll also get access to your country's Command Forum in which even finer, not-necessary-for-the-masses discussions are held.
