When you become vegetarian you are eliminating all kinds of meat but you can still eat animal products like milk and eggs. The process can be fast or slow depending on what you want to eliminate from your diet. Sometimes it is better if you start slow by only eliminating a couple of things first. For example I started only eliminating meat from land animals like chicken, pig, cow and other birds, but I still ate fish for about a year or two, until I became fully vegetarian.

Sometimes I miss out when I see everybody else eating fish, but manage to get other things. On the other hand there is something called beyond meat  where it is vegetable based meat, but it still tastes the same as regular meat, maybe even better. It also helps the animals because we eat the animals, so imagine  being eaten, not fun at all. But you know it does not do much if only one person does it, so if you invite other people to be vegetarians, we can make such a big difference. 

If you still want to make a difference but can't give up meat or fish, you can eliminate it for a short amount of time but still not eliminate it completely. What you can start with is just eliminating beef, or just pork or chicken, it's your choice, and remember you don't have to do this, but if it matters to you then I suggest doing it with someone for fun or not. 

If you eliminate cow meat first it is better then eliminating anything else first because when cows die it increases the temperature and it is very bad for the planet because when the earth's temperature increases all the ice up north melts and causes global warming which increases sea level and for some places around the world it is very dangerous because it can cover land and destroy towns and cities near water across the world. So I suggest you start by eliminating beef first. Think about it, you have the choice to save the world with just one change.