You just have to take a picture with snapchat's camera. Add effect, text, or draw on It and choose how Many time the picture Will be available for viewing (from 1 to 10s). Then, choose Who will receive your snap or add It in your story.
add friends
For adding friends, go on your list of friends and click on the button with 👱 and a +,shearch a user name. When you find who you wanted, click on the + next to the username. You can also syncronize your phonebook with Snapchat.
receive a snap
When you receive a snap, a little red square appear. Click on It and keep your Finger on the screen to see the picture (or video). You can "screenshoot" you friend too.
Your story
When you add a picture (or a video) on your story, your friends can see it during 24 hours. To add a snap on your story, take a picture and when you have to choose which friends will receive it, you just have to click on "my story".