Is this legal? If they don't leave after that, will I then have to start eviction process or just call a sherif?
As a general rule in most jurisdictions, 30 days notice, in writing, will provide you the grounds to start the eviction process--which can take another two or three weeks, perhaps a little longer depending on whether your tenant fights the eviction.
Your best bet is to communicate with your tenant, see if they can work out some kind of repayment plan to recover the lost rent. Sometimes there are arrangements that can be made if the two parties just interface and discuss the matter.
Also, you can get a clearer picture your responsibilities, rights, and duties under Wisconsin law. The state has an excellent website that will provide you with all the information you need to determine what you have to do to evict a non-paying renter in such a fashion as the law is not disturbed.
Check their website at http://wis-law.com/tenant.html
Yes, you can get them to sign a paper stating when they will vacate. It doesn't even need to be 30 days; it can be whatever time period that you can agree on. As the previous answer pointed out, though, you need to assure and protect yourself if you want to begin eviction proceedings. Since the tenant is already in breach by not paying, a 5 day notice must be given in order to evict. If they don't pay, eviction actions may be started after that 5 day period.