How to fix bicycle bearings?

Hi i was noticing my bike's back rim was running like junk so i took the axle out and noticed the ring that holds the bearings in the hub was completely stretched out and was pressed to the top of where the Cogs cassette joins the rim. I had to hammer it out and it snapped in four pieces. I have others but since its on the cog side how do i get the cog off and gain access so i can drop a new one in? I really need to fix this and cant afford to take it to a shop or really buy any special tools.