What can I expect after having smoked weed and failing my drug test? General or dishonorable discharge?

Back in the dark ages (1970s) you would have been processes out of the service as unsuitable to serve. I did several separation investigations as a Captain back then. Things changed a lot. My guess is NJP and drug rehab. Do good and you’ll make it to the end of your enlistment.

DDs come from a General Court Martial and are for the worst offenses. Rule that one out.

I'm no legal scholar but I've only ever seen a Dishonorable or Bad Conduct Discharge after conviction at Court Martial. Those do carry felony convictions into the civilian world. Typically, what I saw over my career was those who popped hot on a drug screen were given Other than Honorable or General Discharge as services usually don't waste the effort on Court Martial for drug use unless there are other violations of UCMJ along with it. General or OTH doesn't have felony status but you'll still staple that to every job application you submit for the rest of your life. Add to that, loss of education and VA benefits as those are tied to Honorable Discharge.

Best case assuming there are no previous disciplinary actions, you'll get Article 15 actions, be sent to a substance abuse program, and with luck retained in the service (at least until the end of your current enlistment). Whether your Commander recommends/approves reenlistment down the road could depend on to many factors to address here.