As you sow, so you reap.
The word karma comes from Sanskrit root ‘Kri’ meaning action or deed, not limiting only to physical but also mental including thought. If there is an action, there is an equal reaction. Nature obeys this law. Every thought, action, desire causes reaction. Ones own karma reaps reaction hence vice brings its own punishment and virtue brings its own reward. There is a definite connection being what you are doing now and what will happen in the future. 
Every human being is threefold in nature, consisting of iccha (desire or feeling), jnana (knowing) and kriya (action). We feel emotions, we know objects like computers etc. and we act on what we want to do or not. The desire gives rise to the thought of achieving something and the sense of achieving when greatly enhanced eventually turns into action. Desire produces karma.
Knowing this, one must sow the seeds which will bear only pleasant fruits. Destiny is your own creation. This is created by thoughts and actions. This requires persistent self-effort. The right choice is almost never the easiest. Be strong. Be brave. Concentrate. Meditate. You can mold your destiny by your own actions. Do not be mis-led by others or yourself.
KarmaYoga is that form of renunciation of attachment and self interest, through which practise, one succeeds in attaining supreme bliss and peace.