Speed, Agility

Hello. it would help if i knew what sport you were taking part in. Going off your 'tags' ill assume its a track and field discipline. If its reaction to something like the start gun then two things you need to do are... improve FOCUS AND POWER

learn to relax and focus on one 'stimulus' i.e. block out the crowd etc. and listen for the gun

also, train for power, eg several sets at low reps and heavy weight.

for more info, join an athletics club, the coaches are all trained by the national athletic body and 90% are ex-athletes themselves.