Most people meditate to relieve tensions and to improve concentration.
Mind when left to its will keeps on wandering. And its these thoughts about what could go wrong, what could be the consequence of your past or present actions that make us tense.
Meditation is the process of calming your mind. Meditation trains you to narrow down your thought process to a single object/thought. When you train your mind to be one pointed this you are more attentive to the one thing you are doing and this increases your concentration and also memory.
The simplest way of meditation is to concentrate on your breath. You can sit in a chair in a comfortable position and concentrate on your breath cycles while you breath in and breath out.
Another way is to mentally chant a mantra/word like OM repetitively. You can start chanting aloud and then slowly fall back to just chanting with lip movement and then finally just chant om mentally without even the lip movement.
There are many more ways but these are simple ways which any one can follow.
You can read Swamy Vivekananda's Raja Yoga PDF which you can get online to understand more about meditation. Raja Yoga is the science of Meditation compiled by Sage Patanjali.
Vivekananda Raja Yoga link :