Help! Masking in illustrator.

For the life of me I can't figure out a RIGHT way to mask in illustrator.

Everytime I mask a shape it screws up the way it looks or it will change its shape a bit. I followed a few illustrator tutorials for masking but nothing seems to work right.

Just create a path/shape of whatever area you want to be masked out. Make sure the shape is at the very top of the first layer in your layers pallete.

Then select everything (ctrl +a) and go object> create clipping mask