If I could give one piece of advice to any player, old or new, it would be always remember the mission. So often these days I hear that competitive players always play to table their opponent, build lists to table their opponent, stomp face regardless of the mission. While I agree that if you are beating your opponent into submission, you are likely winning the game, killing the opponent will not win you every game.
A few stories to make my point: The first of these comes from a year or so ago, I was playing a 1k point game with my Ravenwing army against, Imperial Guard. The mission was capture and control, spearhead. My opponent took the first turn and placed his objective in the middle of his deployment zone. I placed mine behind a hill in the opposite corner of the board, and reserved my entire army to outflank. Turn 2 a few of my squads outflank on his side and start battling with Chimeras, and guardsman taking heavy casualties. Turn 3 the rest of my stuff comes on, one bike squad comes in on the opposite table edge, so I drive it in on my objective out of line of site. Meanwhile he is busy dealing with my bikes, near his objective. At the end of the game I win by contesting his objective and controlling mine. At which point my opponent says “Why did you leave that bike squad back in that corner….”, which was on my objective.
The second story happened just a week ago, I lent a friend of mine (a new player) my Deathwing army to play a 1500 point tourney, I was running Grey Knights. So after winning my first game I go over to see how he is doing. He tells me he lost the game, and I see that he only lost about 4 terminators, while his opponent’s army had little left. He had forgotten that the mission was table quarters, and lost 2 quarters to 1. He went on to finish 4th overall (I ended up 7th), winning his last 2 games. Maybe he could have won the whole thing if he remembered the point of the mission, we’ll never know.
Lastly, more tournaments these days are only giving full points for accomplishing the mission, even if you table your opponent. One more reason to remember the mission, even if you plan to destroy your opponent.