On the main screen, you can sign in with your Yahoo! ID or as Google member. You can also decide to just sign in through Facebook. If you don't have one of those accounts already, you'll have to become a member of one of those sites (click "create account" at the bottom).
Once you're signed in, you can upload photos from your computer to your photostream (your main collection of photos) by clicking here:
Then, they are in your photostream! Voila!
To join a group and start sharing your pictures with others, you can use the "Communities" tab in the picture above to search (or check the bottom of this post for some links). Here's a screenshot of where you click, once you're on the group page:
Once you're a member of the group, you can add photos that are already in your photostream:
Then, you click which photos from your photostream you are adding, and click ADD TO GROUP at the bottom of the right side column (not pictured):
You can favorite any picture by moving your mouse over it and clicking the star in the bottom right hand corner. You can also comment on a picture by clicking the speech bubble next to it.
Groups to get you started sharing:
Our guild group!
Fresh Modern Quilts
The Modern Quilt Guild
Modern Crazy Quilting
Modern Mini Quilts
Quilt As Desired
Don't forget, you can join quilt-a-longs, bees, and swaps through Flickr too! Start searching! If you have any questions or need help, please leave a comment or email me.