I've played scrabble with different groups of people and their rules on first turns vary a lot. The rules I've seen used are:
Which of these are official?
Like you said, the center tile counts as a Double Word Score tile, so whoever plays first gets double the points. That is an official rule.
The second and third rules mentioned are house rules, and are not supported in the Scrabble rules. The second rule would prevent you from hitting one of the adjacent Double Letter Score spaces, as even a 7 letter word wouldn't be long enough to reach it, since they are 5 spaces away.
Even the most recent Scrabble Tournament rules clearly indicate that the first word played must simply cover the center tile, and be spelled top-to-bottom or left-to-right.
LittleBobbyTales is correct about the tournament rules.
Beyond what he has said, (in tournament rules) if the first player for one reason or another accidentally does not make their first play across the center tile and then hits their clock (signaling the end of their turn), the second player has the option to point this out and make the first player remove the play and play again or accept the play. If the play stands, the first play is not doubled, but it is scored according to whatever premium squares that it covers. The second player may then play across the center star, earning a double word score. In this sense, the center star is exactly like a Double Word Score square. This does actually happen occasionally in tournaments, sometimes leading to a rare "triple-double," where the second player plays an 8 letter word from the Triple Word Score on the edge of the board to the center star, through the first word, garnering a six times word score.
It should also be pointed out that it is often good Scrabble strategy to play the first word off-centered from the center star (i.e. contrary to the second bullet point). Tournament Scrabble players specifically study 5-letter words with high point tiles at the beginning or the end in order to play them with the high point tile on the Double Letter Score five squares away from the center star. Also, it is good technique to avoid putting vowels next to the Double Letter Score squares in the rows above and below the center star row (or to the left and right, if the word is played vertically). This avoids the danger of having the second player make a parallel play with a high point letter on the Double Letter Score, garnering four times the value of that letter.
Finally, a number of tournament players play their first word vertically, but the majority don't. It makes no difference, particularly in a casual game, since the board is symmetric, but some tournament players do it to throw their opponent off of their rhythm.
As mentioned, only the first rule is an official rule.
The third rule makes no difference whatsoever, because the board is symmetric.
I find the second (center your word) rule problematic in casual play. It seems like casual players often find it easier to play "to the right, and down", so I try to play my first word so that it ends on the star square. That way, the other players can work in the apparently "more natural" direction.
I think it's much easier for the average person to come up with a list of words starting with K, as opposed to a list of words ending in K, for example.
