Do you need to have the COVID vaccine to join the army?

It is not a pre-requiste to joining but who can say if that won't change. Soldiers are often required to have certain jabs depending on where they are going in the world.
Do you not want to get the vaccine?

No i don’t but if I needed it then I would.

Personally i'd think very strongly about getting it - especially if you say 'if you needed it then you would'. You are protecting yourself and others by having it. No other vaccine ever made has had this amount of research, money and Drs poured into it. Joining the army you work very closely with people. During training you share a room with up to 5 other people, on exercise you'll sleep right next to someone in a shell scrape, during PT you'll be picking someone up. The vaccine is about protecting you as much as it is other people.

I would rather not get it but if the army changes and you have to get it then I will.

Once you joined the army you would most likely be told you'd need to get the vaccine.