Go to your council and get all the info with starting your troop. You will need to sign up for a class through council. Plus get in some other classes now, like with camping and any other things related to activities so you can be "certified" to do them.
I recommend only taking the same grade children as a newbie.
Find a place to hold your meetings. Find out your council rules on that. Decide on a dates to have your meetings.
With Brownies, we always did 1 field trip/activity a month and then met about twice a month for meetings. Some troops are ambitious and do meet weekly.
Once you have the place and dates, then you start to look at the activities that your council has going on. Plus look into parks and other places that have classes just for your age group to earn Try-Its.
Then you can come up with a list of Try-Its that you want to start off with. Since it will be fall, do things outside and then save the crafty type stuff for winter. Once your troop is going you can have the girls help with picking a Try-It to earn.
What we did was give them reasonable choices and the girls would vote.
You also need a co-leader somewhere in there. As part of taking on the troop you need to have some parents volunteer with the troop. You need a co-leader and a cookie seller.
With Brownies, it is imperative that every parent register with the troop. You will need this because generally Brownies are a larger group and you are going to need the parents to help with driving to activities AND staying at the meetings to help AND in addition you need to follow the rules of "Safety Wise" (there is a child/parent ratio you must follow). In order for them to do that they have to be registered with the Girl Scouts.
Kaper Charts are a lifesaver with the little ones. Basically everyone is assigned chores for meetings.
Then you need to plan out your beginning and ending to the meetings. Here is something that "plans out" your first 4 meetings. Great to get some ideas there. You can be simple or as formal as you want. There are flag ceremonies as well as the "Brownie Investiture Ceremony" (Which I do recommend.)
http://www.girlscoutsofpaloalto.org/files/leaders/Brownie/Brownies_1st_4_Meetings.pdf
We had a Brownie supply list that we expected the parents to purchase. Like a box of their own stuff, like scissors, crayons, etc. Back to School sales is a perfect time to ask for that.
They also need a vest and a book is nice.
You need to price out how much all of the patches and pins are plus activities to come up with a realistic amount for a start up cost per girl. Those patches are not cheap when you add it up.
Look over this for starting your troop.
http://www.scoutingweb.com/scoutingweb/Brownie.htm
Anyway there is more but hopefully this helps you out. The more you plan, the easier it is. Good Luck!
Make sure you sign up for a class. The sooner you know the "rules" the better off you will be.