You have to learn the heel command, as previously mentioned.
First, you have to learn to walk before you can run. Always remember, for a pup, it has to be enjoyable and fun.
If possible find a nice field or wooded paths, with less distractions initially, Your the leader of the pack, so always want him to keep up with you, this is much easier with a smaller pup, because you can walk faster than him and he will want hopefully ,to keep close you. Over time, while using the heel command, start getting him closer to your leg. I often pat my leg as the same time as the command, later you can wean him from that.
WIth my pup, i use a longer leash, with hardly any tension on it, so he doesnt feel it, eventually you can apply tension when using the heel command, so he gets the idea that you want him closer. Like all training , do not be excessive with heeling, as it has to continue to be fun.
take it slow, youll be good to go.,