Okay I've talked a bit about the three phases and deployment. What I think is the most important aspect of the game is going in feeling confident you can win. Someone once told me that over 90 percent of games played are decided in the first five minutes while you and your opponent size each other up. If you are playing in a tournament always ask your opponent to show you their army list. Take a good look at the army list and make sure you understand it. If you have any questions then ask.
The ability to have a game winning attitude is very important. It means you fully understand what your army can and cannot do. It means understanding what your opponent's army can and cannot do. You understand the mission rules and you have a plan how to win. Some people say the best made plans crumble during the first turn... It's not true. You play a lot of games and are a student of the game. There are only so many variables and you can boil them down.
Typically if I lose a game it's because I am not that familiar with what my opponent is playing. That's my fault so I have to chalk up the loss to gaining experience. All armies have specific attributes that are common. Special rules are huge. Each army has a certain amount of special rules... Some are very powerul and some are there for fluff. It's very important to be familiar with the special rules. Look at dark eldar as an example... Same set of rules for well over 10 years now but you rarely ever see this army so if you aren't familiar with them it's a huge advantage for your opponent. You always hear how awesome are dark eldar but I think most of the time when you read about how they tore someone a new arsehole most likely the loser had no idea what they were facing. Dark eldar is the most fragile list in the game and they oy have a few good units to choose from. Once you know how to beat them it's a done deal. Daemon Hunters and Witch Hunters are in the same boat... All three races only have one codex. Sisters of Battle and Necrons evolved from a couple of pages of rules back when GW used to put out Chapter Approved articles. They are a big corporation now so stuff like that doesn't happen anymore. You rarely ever see any of these armies winning a big tournament... There are certain armies that steam roller them.
So it is important to be familiar with everything. Preparation is the key to success.