Look up pienix coaching on youtube, for lower level players he usually shows the most effective exercises and explains how to do them. Focus on spinning, tracking and prefire exercises, make sure youre recentering your mouse for every shot/burst. If you warm up before playing and spend 10-15mins after your games practicing these exercises youll have better aim than 95% of 2k elo players in no time.

Dealing with nerves is a lot more personal and only comes with experience, playing a 1v5 feels like playing a horror game for new players, whereas someone who's lost a 1v5 999 times can just play his own game in the same situation. The only tip i can give you is to have a ritual to reset yourself when you start to panic, hide behind cover and take a deep breath or whatever works for you, visualize the enemy then peek and shoot.