Hello Leah,
Myeloma is very treatable and people can lead a fairly normal life with it. If your dad is being started on treatment he will be monitored closely and any problems will be dealt with straight away. My husband mainly had allergic reactions which caused upper body rashes, easily dealt with by tweaking his medication.
At the start of diagnosis you and your dad might feel overwhelmed by it all, there might be a seemingly endless round of appointments and daily tablets to get used to. It does settle down and becomes routine. My husband used to hate taking tablets, wouldn’t even take paracetomol, but he now accepts it is a very small part of his day.
There are lots of booklets you can download or order from here which help you understand what is going on.
All the best
Teresa