When you purchase rulers, buy the same brand as there are many on the market. Some are not as accurate as others.
Your rotary cutter should be a 45mm.
Also, I would suggest you join a quilt guild. There are many folks in these groups who will help guide you. Guilds are also an avenue to workshops where you go and can get one-on-one teaching.
Welcome, new quilter. Blessings, Suzanne