There are several ways this can be done and only a few of them involve any actual hacks.
The key thing to know is where they are actually coming from.
They could be coming from your computer, in which case you are infected with malware/virus/trojan/worm/etc.
They could be coming from your email account (if a web based email…Gmail, Yahoo, etc) in which case there’s a chance that someone got the password to that account/took over it/etc.
They could be coming from anywhere and are just using your email address as the ‘sender’...this is the most common and since you are listed as the sender, you are getting the bounces, as well as the blame.
If you are using an ISP provided email account, contact them to see if the garbage actually was sent from your account. If you are using a webmail, like Gmail, then you need to change your passwords (use another machine to do so, at least until you make sure you are not infected with anything).
Then, while waiting to find out where they came from…scan your computer with at least one AV and something like MBAM (Malwarebytes AntiMalware).