A STAR SHALL COME FORTH= (Old fashioned) A star will appear (forth = forwards).
This is a quotation from an ancient prophecy in the Bible referring to a new star which would appear at the time of the Messiah's birth, so the appearance of that new star announces the birth of Jesus, the Messiah (the saviour of the world). This new star was a sign and also a guide for the three wise men (or "magi") in Persia who saw the sign and travelled to Bethlehem in the direction of the star to see the newborn. The names of the Three Wise Men were: Melchior, Gaspar and Balthasar.
Some people think that star was a comet that appeared then (comet Halley), others think the new "star" was the result of a conjunction of three planets that happened at that time too, so when they were joined together in the sky they had the appearance of a new bright star.
We know today that Jesus was born a few years earlier than we thought, probably in 6 or 7 BC. In the evening of Feb. 25, 6 BC (year 6 before Christ) Mars, Jupiter and Saturn joined together in the sky in the constellation of Pisces.
Another possible explanation for the Star of Bethlehem is the three-times passing of Jupiter and Saturn between May and December in 7 BC; a rare triple or "great conjunction." Jupiter appeared to pass one degree north of Saturn on May 29; practically the same on Sept. 30; then finally a third time on Dec. 5.
There is no doubt about the visibility of these events, mostly opposite to the Sun in night-time skies. As for their astrological impact, the Magi would have certainly noticed that both planets did not appear to separate widely between their conjunctions. In fact, for eight consecutive months the time it might have taken to travel the 500 miles or more from Babylonia to Judea Jupiter and Saturn remained within three degrees of each other, from late April of 7 BC until early January of 6 BC.

REJOICE= (Old fashioned) Be happy.

GOOD TIDINGS= (Old English) Good news.

UNTO YOU= (Old fashioned) To you; for you.

THE CITY OF DAVID= Bethlehem /beθlɪhem/. This town is called "the city of David" because David, the most important of the kings of Israel, was born there. The prophecies said that the Messiah would be born in Bethlehem and would be a descendant of king David. When the Roman emperor ordered a census, all the men in the empire had to go to their home town to register, so Joseph (who was living in Nazareth with Mary) had to go to Bethlehem to register. There were so many people in that small town that they couldn't find an inn (a kind of hotel) to stay. Someone let them stay in a cave that was used as a stable for the cattle, and there was where Jesus was born, as you can see on this video.

CHRIST= /kraɪst/ This word is the Greek translation of the Hebrew word "Messiah" /məsaɪə/, which means "the anointed one". In ancient Hebrew (Israeli) tradition, the person chosen by God to lead the people was anointed (sacred oil was poured over his head), so the idea of "Messiah" is "the one chosen by God to be the leader, to save the people". In the beginning the correct expression was "Jesus, the Christ" (Jesus, the Messiah), but soon it became a kind of surname and we say "Jesus Christ" and we can say simply "Jesus" or simply "Christ". In some languages both names have merged into a single one, like in Spanish, where we can say "Jesucristo".

THE LORD= God.

WRAPPED= Covered in, with a paper or cloth around.

MANGER= /meɪndʒə*/ A square box with straw for animals to feed. As there was no bed or cradle in the cave, when Jesus was born they placed Him in a manger.

I HAVE BEEN GIVEN THE STRENGTH (THAT) I PRAYED FOR= This is a passive sentence. In English, if we have a verb with a direct and an indirect object, we can transform it in two different passive voices, depending on what we want to emphasize:
1- Someone gave me a present (active voice)
Here I'm talking about SOMEONE, which is too indefinite to be important.
2- A present was given to me (passive voice 1)
Here I'm talking about the PRESENT.
3- I was given a present (passive voice 2)
Here I'm talking about ME (I).
So this sentence is a passive voice 2 and means: I prayed (to God) and He has given me the strength I needed.