RPGMemes

Protip: Literally just do it, because the easiest and fastest way to learn how to DM is to DM something and then think about how you DMed it.

I would recommend an official module to you as a way to help you learn how to balance encounters, but you must be aware that most if not all official modules are actually critically flawed in such a way that negates the main benefit of DnD: A pre-made adventure can only allow your players to follow one path.

All told I think the pre-made adventure that's the least bad is Ghosts of Saltmarsh, followed by Hoard of the Dragon Queen, but these both have VERY rough level 1 adventures, so honestly just make your own. You can adjust things like enemy HP and hit modifiers on the fly if you realize you've made a horrible mistake (just don't tell the players).

Oh, but my one key piece of advice? If you want to run an adventure your players are going to love? Simply ask them what they want out of the adventure, and react accordingly. If they want loot, then give them coins, gems, and unique valuables. If they want an overarching plot, make up a villain and have their minions be thematic. If they want to meet dragons, introduce one early and make the players meet them and some other dragons who know them a few more times throughout the adventure. DnD's greatest strength, by an absolute mile, is the ability to tailor content exactly to what the players want. No video game can do that.