What do I need to do to get guardianship of my mentally disabled brother?

I am very sorry for your loss, and I congratulate you for taking care of your brother. May be there is in the town Hall a social worker who can help you, or you go to the Court, where they should be able to tell you what you have to do and where. In any case, don't wait too long. I have been guardian for my husband who had Alzheimer, being just over 50 years. Here with us, everything was arranged by the Court. But of course legislation is different every state / country
Lots of strength and greetings

My condolences on the recent passing of your mother. She is indeed with angels for caring for your brother. Go to your state's superior court where the probate division is (it might be different in your city) and file a guardianship with them. Make it urgent, and usually a waiver for court costs is available if you do not have the funds. You will need to go to social security with him and get a "Representative Payee" status for his social security benefits. Notify them ASAP. God bless you both.

Wisconsin has a Guardianship Support Center through the Greater Wisoconsin Agengy on Aging Resources. (GWAAR)
http://www.gwaar.org/for-seniors-and-families/elder-law-and-advocacy-center/wisconsin-guardianship-support-center.html
It would be good to know if she appointed anyone upon her death.
You can also call the Dane County Aging and Disability Resource Center ( ADRC)
608-240-7400
We have information and resources for persons with disabilities.
Good Luck

Can't tell from your question if your brother is developmentally disabled or suffers from mental illness. I would Google arc Wisconsin and find a local chapter of the Arc that can help you find resources, lawyers etc. in your area that are familiar with issues for people with developmental disabilities and NAMI Wisconsin for the same set of information if you are dealing with mental illness.

So sorry for your loss. The only thing you need to do is to go to a lawyer and start the process of getting guardianship for your brother. Ask about services for him and any kind of aid, financial etc., that may be available to you and/or you brother.
If your Mom left no will, look into life insurance etc. that she may have had. Definitely start with a lawyer. Good luck and take care!