Does the concentration occurs during first jhana?or is it relates to the samadhi which involves in second jhana?What is actually one pointed of the mind means?
Concentration is part of every mind state: the normal, regular, every-day-of-life mind states; the unwholesome ones; the wholesome ones. It is absolutely not limited to jhana states!
One-pointed means basically that it focusses the entire mind on to an object. In vipassana we speak of momentary one-pointedness where the mind in this moment focusses entirely on this object and the next moment on the next object.
"Mindfulness" means to "remember to apply" and "not forget" the Dhamma teachings. To quote:
What is sammasati? Sati means to bear in mind or bring to mind. Sati is the state of recollecting, the state of remembering, the state of non-fading, the state of non-forgetting. [Vbh.105, 286]
One is mindful to abandon wrong view & to enter & remain in right view: This is one's right mindfulness. Thus these three qualities — right view, right effort & right mindfulness — run & circle around right view. MN 117
For example, in Buddhist meditation, it means to always remember to keep the mind free from craving. The suttas (MN 10; MN 118) say the meditator: "is mindful to abandon covetousness & distress in relation to the world".
"Concentration" is the state of non-distractedness or singular preoccuption; which occurs when the mind is continuously mindful. Concentration is "samadhi' ("collectedness"); when the mind gathers its potentials onto a primary task and objective.
Every jhana has one-pointedness/singleness of mind; as follows:
The first jhana has five factors. There is the case where, in a monk who has attained the five-factored first jhana, there occurs directed thought, evaluation, rapture, pleasure & singleness of mind. MN 43
Unflagging persistence was aroused in me, and unmuddled mindfulness established. My body was calm & unaroused, my mind concentrated & single. Quite withdrawn from sensuality, withdrawn from unskillful mental qualities, I entered & remained in the first jhana... MN 19
And what is the faculty of concentration? There is the case where a monk, a disciple of the noble ones, making it his object to let go, attains concentration, attains singleness of mind. Quite withdrawn from sensuality, withdrawn from unskillful mental qualities, he enters & remains in the first jhana... SN 48.10
One-pointedness is when the mind becomes glued to a mental image.
Both the 1st and 2nd jhana have one-pointedness however in the 1st jhana, while the "centre" of the mind is unmoving; the outer parts of the mind (consciousness; not thoughts) still move or 'hover' around the factors of jhana; similar to how a wheel is fixed to an axle but spins around the axle. Refer to Ajahn Brahm's teaching about the "jhana wobble".
On Buddhist chatsites on the internet, there is not one person who has declared they have attained jhana who has actually attained jhana. These people are always overestimating their experiences.