Assuming you've been paying exactly zero attention to anything going on, here's a few major things that he is responsible for:

Travel ban targeting Islamic countries

Withdrawal of Paris Agreement

Fired head of FBI when Comey wouldn't declare personal loyalty

Issued a directive to the department of health agencies "waive, defer, grant exemptions from, or delay" portions of the ACA

Soon to be pretty closely tied to:

Tax bill making graduate school significantly more expensive

FCC votes to kill net neutrality next month

These are, in my opinion, the worst offenses. There are many other things that he is either responsible for or closely related to. A simple google search yields list after list of things he's done both large and small.