This is a really simple plugin. You don't need to do any coding unless you've already seriously altered wordpress. The problem with the plugin is that there is no basic instruction manual for it. So, here's a simple guide:
1. Install Contact Form 7 plugin, and activate it.
2. Install Really Simple Captcha plugin and activate it.
3. You should see a "CONTACT" button on your wordpress dashboard, between "Clients" and "Appearance". Click it to open the control panel.
4. Click Add New button to start a new contact form, and choose your language from the dialog box.
5. YOu'll see all the tags there for a basic form. Click the "Generate Tag" button. Near the bottom, you'll see the CAPTCHA menu item. Select it.
6. YOu'll see the CAPTCHA control panel now. Select the size you want. You can just leave all the defaults and it will work. If you want a differennt colored captcha image, change the foreground and background colors by entering #xxxxxx style color codes.
7. Copy the 2 tags that are generated in the bottom of the box, and paste them into your form. Name your form at the top of the page, and click the blue "Save" button.
8. In the box at the top of the page, you'll see the tag generated for the form. (this has the captcha box embedded in it.) Copy it, and then paste the form code wherever you want, on a page, a text widget, a post, etc.