Hey! And welcome to the forum! Many of us also evolved into meadmaking from our beer brewing days, so you're in good company.
First off, I would recommend you check out the resources on the FAQ's page, here
http://www.gotmead.com/forum/faq.php?faq=meadfaq#faq_newbee, especially the Newbee's Guide to making mead. This will answer many of your questions.
Additionally, for your first mead, I strongly recommend a JAO, Joe's Ancient Orange. It produces very good results right off the bat, with easy to obtain ingredients, and very little fuss. There's a HUGE thread on this forum about it, like 1500+ posts long, but you'll find the actual recipe in post #2.
If you decide to go off on your own and make something of your own creation, or something from a recipe you found elsewhere, please come post the recipe here first, and let the experienced mazers take a look. Many come halfway into ferments gone wrong, asking advice, and there is much less to be done at that point to rescue what could have been a good batch.
On the topic of gas, or purging oxygen from the top of the carboy, you'll find varying opinions, but I would wager 90% or better would say don't worry about it. It takes far more exposure to oxygen to "flaw" a mead than a little headspace. Just do what you would normally do with a beer to limit oxygenation during transfer and storage.
As for nutrients, again you will find varying methods espoused by those who have developed their own preferred regiments. Generally you'll find staggered nutrient additions recommended at 24 hours after the pitch, 48-72 hours after, and at the 1/3 sugar break (when 1/3 of the fermentable sugars have been consumed, measured by gravity readings). And a variety of nutrients are available, with the most common being Fermaid-K, Fermaid-O, Fermax, and pure DAP (diammonium phosphate). I personally have gone to the TOSNA nutrient protocol, which uses only Fermaid-O (an organic nutrient). The TOSNA protocol is completely spelled out at
www.meadmaderight.com. I have had nothing but good results when I follow it.
That's probably enough for now. Again, welcome, and please come back and share ALL the details of your recipe before you kick it off. And get yourself some GoFerm and a hydrometer if you don't already have them. They will be highly highly recommended at nearly every turn in this forum!
Cheers!
