Some regard Buddhism as a kind of belief. Belief also means faith. Of course faith is needed in Buddhism, but it would be oversimplified to regard Buddhism as a belief since keeping faith is only one of the aspects of Buddhism.  The foundation and the priority of Buddhism are not about belief, but wisdom and compassion.  

- Quote from The Right View, "Buddhism—the Definition"

If it is liberation that we seek, we must plant the seed of liberation, which will then yield the fruit. Such is the view of Buddhism.

Under normal circumstances, what we do now, either good or bad, definitely will affect future karmic results but not quite so imminently the manifestation of karma at present. However, exceptions are possible with special circumstances.

Knowing virtuous karma is, like dreams, intrinsically illusory. If we can contemplate in this way, even if anger arises, it cannot destroy the root of virtue.

Buddhism holds the doctrine of dependent arising of all phenomena or compounded phenomena. What is dependent arising? It means that cause begets effect. All phenomena are the manifestations of dependent arising, the results of conditioned genesis.

The complexity of the cycle of cause and effect and how it passes through the past, present and future make it possible only for the Buddha to comprehend entirely its causal relation. Others merely glimpse different parts of the cycle.

If you would like to know more about karma, you can read the fourth chapter of the Abhidharma-kosha-shastra, which clearly explains the workings of cause and effect.

If we have neither attained any realization nor dedicated merit, but are constantly filled with anger, virtuous karma will be destroyed very easily. For ordinary people, the best way to save accumulated good karma is dedication of merit.

We can be as virtuous as we would like in this life, retribution may still await us if we cannot purify all our negative karma of the past. Once this type of karma matures, there is no escape but to bear its effect albeit temporarily.