I often fly out of the tournament at the beginning! What could be the reason?
If you have trouble when the blinds are low and stacks are deep you need to learn more about pot control. Try watching some Twitch streams, some MTT training videos, or post some hands here and people can help you out with any questions.
Basically pot control is checking a street (flop, turn, or river) with a strong hand since you are unlikely to be ahead should your whole stack go in on the hand. If you bet every street with top pair and go all in on the end without improving, competent players will call with only hands that beat you. The more you play the better you will get at knowing when to continue betting with one pair hands and when you should be checking.
There are similar problems with re-raising pre-flop early in a tournament and getting all in with a hand like AQ, AK, TT, or JJ - for a deep stack against most players you can be well behind their all-in range. Some deal with this by only playing monsters and miss out on many good opportunities to make chips in the early levels.
You can play suited connectors and pairs for a small investment and get good value when you connect with two pair, trips, straights, flushes, and sets. As you get more comfortable with these hands you can try picking up some pots with bluffs and semi-bluffs when other players are all weak.
Best of luck to you!