Q.
Dear Rabbi,
I'm interested in investigating the way Jews prayed during that times of King David. 
(When I pray) I really doubt the liturgy in the three Amidot we say every day, even Rambam says that Arbit is optional.
I assume, Shaharit and Minhah was from the last Sanhedrin of the modern era year 400.
Perhaps Shlomoh haMelech just prayed once in the morning (communal prayer in his Temple) or when he was alone?
Must we distinguish between communal and personal prayer?
Communal prayer must be only done in plural (we) in Lashon-Qodesh?
But every Jew may pray his personal prayer in street-language - Aramaic without any formula, with his own words from his heart in singular (me)?
Non Jews can pray his native language to understand prayer meaning? 
Please, can you make a video or audio about this?
 
A.
Before the end of the second temple to my knowledge there was never an order to the prayers but rather people just faced toward Yerushalayim and meditated.
This practice has really been limited  to Tachanun now a days which is the portion where we are supposed to add our own personal supplications..
And this now a days has been even been replaced for a standardized text.
Some even doubt if the Shema in the form that it is said today was even recited in ancient times.
As you know that its only a legend that the each of our patriarchs compiled the liturgies within the times of prayer.
 
Prayer as its understood today was instituted to help us remember the temple services our land and our Creator..
Even if King David himself would of instituted something it would not make it any more viable than if a later the Sanhedrin would of established it.
 
Regarding the text of the Amidah itself these may have been created even prior to the destruction of the Temple and only later were incorporated into the framework of the compiled prayer known as the Amidah.
 
Regarding our Siddur, we have only used sources dating to before the 13th century which in turn has produced the most halachicly sound concise siddur on the market. (Download yours free on our homepage)