When you're 7 you will move up to Brownies. However, you may need to start looking at which unit you want to go to when you're 6 as some units are very popular. Here is a bit about what Brownies do.
When you fist join you will get a Brownie buddy to make sure you're not 
left on your own. here are many other exciting things you get to do as a Brownie such as:
Choose the Brownie clothes you wear.
Be part of a special group of friends called a Six.
Play games and do all sorts of exciting things outside.
Hold a Pow-wow to decide what to do.
Get badges for things you really love doing.
Have a special book telling you what being a Brownie is all about.
Why not take a look at Tuesday Brownie's website to see what else you can do at Brownies:
or how about looking at or districts website which has more information on Rainbows and Brownies