New To Advertising on the WEB

ok so i have a small clan gaming website and thought i would try google adsense out before getting into some bigger websites. So after a week or two i've actually earned some money. But i notice the eCPM fluctuates a lot. I notice its gone lower as the week has gone on. What determines how much iam making, and do u have any tipes or suggestions for me regarding adsense. Also iam looking at making a much bigger site so iam wonderin what other ad options i have. thanks