To answer first part of your question:
How to insert google Ads in my site:
Steps:
1. Goto
https://www.google.com/adsense/
2. If you already have google/ gmail account then use that to login
3. Else create new account.
4. Read the policy at
https://www.google.com/adsense/supp...&sourceid=aso&subid=ww-ww-et-asui&medium=link
5. Adhere to the policy as Google Adsense is very ban happy, they will ban your account for any violation of policy.
6. Once account is ready go to Adsense setup where you can customize (post again here if you need help in that)
7. Take that code and paste in the appropriate place in your site.
Second part:
Yes you can start integrating the code when you are developing the site though Google Adsense stream ads based on content on the site so initially if you do not have much content, you may see only public ads (if you have selected that option) or empty space (if that option is selected).
For layout and how it appears you can check at my site
http://ipo.investorsouk.com/
Hope this help.