Now this sounds pretty familiar for me.
Congratulations, it's a hard step and it surely brings more joy to choose when to "have fun" and do stuff, rather than such stuff picking you to do it. As you've said, it's not as physically dangerous as the hard drugs, but still manages to dent your willpower.
Anxiety will be pretty hard to cope in the first months, and if you're not a good dwarf who
can handle stress easily then you might be prone to fits of anger and loss of sleep.
Being off from your own mind is a must, as Spaghetti7 said, and doing a lot of physical exercise helps with this, and will get you asleep while your normal sleep rythm gets on track. My idea of physical exercise has to be understood as doing really demanding stuff, like swimming or getting on a bike and get to the countryside and back, because skateboarding or just walking allow a greater degree of thinking, and is just not enough for your body to express itself. There will be a lot of things to express that were just under your smoking habit, expect to lose yourself at times. There's really no need to say this, but you might get pretty fit and healthy, not that you might care but it's helpful.
Claim your old hobbies, play videogames or watch TV when your brain gets overclocked. It's one of the reasons that there are TV sets at mental institutions, they let your mind at ease to go away. It's heavily recommended that you do not practice zapping while watching TV.
Build new habits, out of nothing, so the relationship between your idleness and the environment gets rebuilt. Wash your face every night, write a journal (even better if you use pen and paper) and consign some basic goals for the day. Even if you don't accomplish them you might have some kind of lead for the daily routine.
I see your brother is involved. I hope he doesn't smoke, but anyway, for the time being don't stay in places where people smoke, even if you feel determined and say "no" to any weed offering, doing just that might take your anxiety up a few notches. Remember that willpower is limited, do not abuse that plentiful resource, you will need it. Getting back to your brother, talk to him and ask some patience his side, you'll do better in company for these first days, as you gather more reasons to be confident and certain about your decision. Also, your body has to clean all that waste while your sinaptic links recover their original strenght, somebody who's just there for a pillow-talk or to get a medical check-up is always helpful.
Drink water. Lots of it, for the aforementioned reason. That'll ease the process and peeing happens to be also a blissful routine; as Bauglir said, if you think that leaking the chemicals off from your body is helping you somehow then you should have more reasons to feel more relaxed.
As a small endnote, if you're somehow tied to a lot of social media websites as Facebook or Twitter, to name a few, you might do better if you close those services for a while. While it sounds a bit nutty (nothing really does when you're under a panic attack) part of the anxiety stuff derives from your perceived image and what you think you're reflecting to society as a whole. As people behind screens tend to be quite judgemental and tactless, then you should do better without them for a while. Take your time to rendezvous with non-smoking friends, face to face, and do some stuff together a day in a week. I think many of them will understand your choice and will be pretty supportive.
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Maybe I went way over the original question, but I hope it helps somehow, I've seen this happen a few times. Tell us how it comes.