How can you find problems to do?
You can view past contests here. You can just select any contest, find the division you are looking for, and select “View problem”. Keep in mind, I do not think you can view contest problems before the 2015 season.
Hope this helps!
If you are looking for USACO problems, then I recommend looking for problems here.
However, if you are open to solving non-USACO problems (which still help you develop your skills in the corresponding topics for each division), then I recommend looking for problems here.
Hopefully the instructions on how to filter problems by difficulty and topic are self-explanatory. If they aren’t, then feel free to ask.
Hope this helps! Good luck.