I'm sure you've probably already read this, but at 12 weeks old, your pup is to young to be going for long walks. When Riley was that age, we basically did short (and I do mean short) strolls up and down our street at HIS pace (my pace would be no more than 10 minutes). If he wanted to stop to sniff something for 10 minutes, that's what we did. I didn't pull him because I wanted him to have fun just thinking and exploring new sights and smells.
To get him used to walking on the leash, I first let him walk around the yard with the leash dragging behind him. Of course, I supervised him because I didn't want him to get caught on anything. Once he was comfortable with this, I picked up the leash and again let him go and I simply followed.
Eventually we got to the point where he had to learn how to walk normally. For that, I used some treats to bribe him. Every few steps forwards he was rewarded with a treat. Being a smart boxer, it didn't take him long to catch on that.
Just keep working with your pup on short walks (I personally would not go any longer than 10 minutes at this age) and keep it fun and rewarding (to your pup). Don't drag him because I highly doubt that would be his idea for fun. Be sure to have plenty of yummy treats on hand as well, it'll help. Good luck!!