1. Pyramids are the massive structures found in Egypt.
Yes, The giant pyramids of Egypt are known to everyone. No, they aren't the only ones, few people are aware of the fact that the Americas have more pyramids than the rest of the world combined! The largest Pyramid in the world is the Great Pyramid of Cholula, Puebla,Mexico
Some other pyramids
'Monk Mounds' in Illinois,USA.
Pyramid of the Sun,Pyramid of the Moon Mexico City, Mexico
Pyramid of the Inca, Cuzco, Peru
The Yucutan Pyramid (Chichen Itza).
Our ancient Hindu Kings were similar to Sun workshipers and most of our Hindu temples have the major structures (Gopurams) that remsembles a pyramid(wide base and tapering top).
Pyramid of Cestius, Rome although small, still exists while the larger pyramid of Romulus was taken apart around the 16th century.
Extant Pyramids of Sudan ( more than 200 small pyramids)
The Pyramid is a powerful symbol in many cultures and are depicted in religious texts,flags, seals also. The US dollar bill has an unfinished Pyramid with an eye at the top, it literally means that 'God is watching over (humans)'. It is a symbol used by Freemason society, on the official seals of many European countries.
2.What you know about the mysteries of pyramids?
There are many conspiracy theories, theories linking pyramids to powerful ancient brotherhood sects etc.
Most often, it would be built as a burial chamber, to enhance the relationships between Gods and the Kings. While the Epyptian pyramids were often a tomb for the royals, the other's in Americas were actually for Gods.
Some historians beleive, Pyramids are not just resting places, they are places that were part of their daily lives.
3.I would like to know who built it, how they built it and for what purpose.
the above answers give you the details
4.Why they were positioned to a particular star system at the time of construction?
The well known theory of Giza-Orion Correlation theory correlates the three big pyramids and the three stars in the Orion but this has several critics.
5.Are there any relations of the dimension of the pyramids to that of the planet Earth?
Again the mega size of the pyramids has made many scholars link the size to that of the Earth and came up with the popular 1/43200 ratio. The square base circumference is equal to half of one minute arc at the place of the equator
Can I get a scientific answer?
As far as Pyramids go, It is difficult to separate theories, beliefs and logically reasoning . It up to to you to believe in what sound correct to you.