TDM - Team Death Match is a game mode similar to FFA but with teams. The goal for each team is to kill the opposing team and stay alive. The number of kills that each player makes is added to the total of their team's kills which includes all the kills made by the team. The game ends when either a team reaches the score limit or a timer runs out. Both, the score limit and the round time are set by the person who hosts the server.
TDM is just like FFA except that instead of winning being based around kills, winning in TDM is based around net kills. Net kills are your kills minus your deaths. In other words, the way to be an asset to your team is to kill more than you die. Getting many kills is useless if you also get lots of deaths.
1) Play for the team. Try to protect teammates and work with them to kill more than you die. If you work alone, you will be killed far more easily, which makes you a burden for your team. It doesn't matter if you have the least amount of kills on your team, as long as you have net kills.
2) Bars: Get bars. Much like in FFA, bars can be vital here. When you take down opposing planes, it gives you colorful bars above your plane, ranging from 1 bronze (worst) to 3 golds (best). These bars boost your strength, speed, agility, and armor. In this way, you can kill more easily and stay alive more easily. This is essential to easily rack up net kills.
3) Don't mind getting assists. Unlike in FFA, where you have to get kills to count and stealing others' kills helps you win, in TDM assists are just as useful. When you get an assist it means that one of your teammates took a plane down, which adds the same amount of kills to your team's score as if you had killed it. In other words, don't be selfish, just shoot planes as much as possible.
All in all, TDM is all about staying alive while helping your team to weaken and/or take down the opposing team's planes. It's a game mode that truly requires selflessness in terms of kills, because you're working for the team, so you must be smart, even if you aren't the best player on your team.