What backpack should i get for my first backpacking trip

hey everyone! I‘m planning to go to asia soon and I am thinking about ditching my suitcase and getting my first backpack. Is there anything i should be aware of before buying a backpack? Is there a brand u guys would recommend me ( preferably on the cheap side)? How many litres in terms of capacity, do I need for a 3 week trip. ( i am NOT a light packer unfortunatly…)

I am very grateful for each and every tip :)