What to do to avoid time trouble in classical game?

I'm a youth player who always, in classical games, gets in time trouble. What should I do to avoid this?

Practice Tactics. A lot.

Why tactics? I said time trouble.

It will help in the long run. The more you practice, the quicker you will see them

Practicing tactics made me slow. I always got into horrible time trouble OTB. But the way I practiced them led to that.

It sounds simple, but the answer is often that practice should mimic the performance. In other words whatever you do in practice is how you can expect to preform.

If you solve tactics with no time limit, or if you play a lot of blitz, or if you play a lot of correspondence, etc none of this builds the kind of calculation habits you want for an OTB tournament game.

First of all, figure out the pace you want. For the sake of simplicity let say the time control is 60 minutes and you want to reach move 40. Well 60/40 = 1.5 so on average you want to be playing about 10 moves every 15 minutes. On move 20 if you're below 30 minutes you know you need to speed up.

Of course some moves are easier than others. So expect some moves will take less than 1 minute, and a good rule of thumb IMO is never* use more than 10% of your time for a single move. 

* I said never, but maximum twice a game is ok if it's a critical tactical position

So how do you train that? How do you build good habits before the tournament game starts? You can think up ways yourself, but for example, solve tactics but give yourself a time limit of 10% of your classical game. If the time control is 2 hours, then your time limit is 12 minutes. Try to get puzzles that are difficult enough you need that much time.

You can also play over GM games. Cover up the moves and play guess the move. For "easy" moves set a time limit of 1 or 2 minutes. For positions you feel are tactical again give yourself 10% of your tournament time control.

The point of all of this is you want to be sitting at your tournament game and have a little timer in your head that lets you know you should be wrapping things up soon. You want habits that make you calculate the most important things first, and you want the good sense to settle for good moves and not burn your time trying to find the best move. You're only allowed to try and find the best move once or twice during a game. Yes even in classical, because even a 5 or 7 hour game is far too fast to strive for perfection.

Also learn common opening principles and strategical elements. This way, some moves like developing your pieces or maneuvering them will seem simpler and you will be able to play them almost automatically. When I meant tactics I meant those on the tactics section on chess.com. It gives you a target time to solve the tactics. If you can solve all of the tactics within the target time (which I think is pretty easy) you have good tactical ability. Then, start pushing yourself to solve them 3, then 5 seconds faster than the time limit. Play a lot of puzzle rush, when you get a score of 20 you will be pretty good. I'm trash in puzzle rush but thats because I get really stressed while playing it. That's another important point: keep being calm at all times. Stress worsens your tactical ability, and you will lose much more time. If you think you're tight on time and get really stressed, consider going to the bathroom and pouring some water on your face to concentrate. You might lose like a minute but if you can afford that, it will help you throughout the match

Play faster.

Thank you all.

1. Focus on your opening preparation. If you spend like 40-50 minutes in the opening it's not good
2. Improve your calculation ability

I've seen a lot of chess players that get into time trouble and it's often because of one of the following things:
1. Perfectionism - They want to make the perfect move and take all the time necessary to be comfortable making that move regardless of the fact that time trouble later on will force them to make far less perfect moves.
2. Indecisiveness - They have a couple of options, but they can't choose between them and are afraid to make the wrong choice.
3. Tactical fear - So worried about making a blunder that everything is checked a 100 times to make sure they don't miss a tactic.
4. Endless calculations - They find the right moves, but want to know what happens after that, and after that and after that, taking endless time calculating.
5. Procrastination - They know they need to make a decision, but they still put it off and get into a rush when time runs out, equal to studying for an exam a day before the test instead of taking the time earlier.
Which one(s) are you? The solution is tied to the cause I guess.
If you are a perfectionist and burn all your time on that, try to become more practical. Identify the move that is good for sure and then do a quick check to see if there is a better one, but make that check quickly and if you can't identify a better one within a certain (short) time, just make the good move you had already identified and accept the fact that it is good, but perhaps not perfect if you analyse it afterwards.
If you are indecisive, kick yourself in the butt and just make the move that feels best. If you weren't able to distinguish the advantage of one to another, the difference is often not that much.
If you have a tactical fear, try to study tactics every day (puzzle rush, battle, tactics trainer, etc. etc.). That way you become better at tactics and can lose the fear somewhat.
If you are calculating endlessly, realise that you can never calculate everything to the end. In this case you have to make a practical decision after a certain amount of time. Either you trust that the calculations should work in your favour or it is too tricky and you make a more safe move.
If you procrastinate, then you need to work on time management. When there is only one move and one move only, make that one and think/calculate later, there is no need to waste time on a forced move when you can calculate in the opponents time. When your opponent has only one or two choices, try to stay behind the board and calculate the follow up, so you might have your answer already and can move right after he makes his move, putting time pressure on the opponent. And lastly, use time only to calculate/decide in difficult positions, as reflecting on an easy position or thinking back at earlier moves is a waste of time. When you use your time (and your opponent's time) wisely, make more practical moves and only calculate when you think it could give you a clear advantage, you will probably end up with more time in the end and a better position to show for it. 

Hope it helps!

Thank you so much @Rookochet!!

Isn't a classical game 90min? Who would have time trouble here?