Google payment options are divided into two types: Postpay and Prepay. One or both will be available to you depending on your country and currency. Note that you won't be able to switch from a postpay option to a prepay option or vice versa, so be careful when selecting your payment method as you set up your account.
Google Adwords Basics
The example below shows how a High Quality Score can result in a higher ad rank.
Quality Score is a measure of how relevant your advertising is for users. Each of your keywords gets a Quality Score, and the score is determined by how well your keyword matches what users are searching for. Quality Scores have a rating from 1 to 10, where 1 is the lowest rating and 10 is the highest. In general, the higher your Quality Score, the lower your costs and the better your ad position.
There is No minimum spending for AdWords, and no minimum contract requirements or other "lock-in" rules apply. You have complete control over the maximum amount that you want to spend per click, impression, or day. You can also pause your campaigns or cancel your account at any time.