For a new GM I would recommend starting with an episodic feel with a series of episodic adventures - which can be changed/amended according to player activity in earlier adventures, stated preference etc - growing over time towards an epic confrontation. I would therefore tend not to recommend an AP, which in my experience take a lot of practice to use well. The big risk with an AP is that players feel shuttled down a pre-set path with no real input. They can also place a heavy burden on the GM in understanding how it all fits together, though more episodic APs like my current Shattered Star one mitigate this.
The Primeval Thule campaign setting book is excellent IMO and would be a good basis for such an open campaign. It has adventures in it, but you can also drop in many published dungeon adventures as-is; Goodman Games stuff fits the swords & sorcery flavour but I wasn't impressed by the one GG 5e adventure I bought. Lost Mine of Phandelver would need some conversion to give a Thule feel - goblinoids become Beastmen or savages, use a Thulean dragon not the regular green dragon etc. I would generally think it easiest to use the adventures in the Thule setting book plus the NPCs, monsters etc, give the players choice in what to do, riff off that, and prep a few extra encounters you bring each week that you can always throw in if unsure what to do - "Bandits Attack!" Think about what excites you most, think of a villain or group with long term plans, and start introducing them over time. As your skills develop you can create your own adventures, following the DMG guidelines on monsters and treasure. You can draw your own maps or get free ones online; Dyson Logos' site is particularly great - https://rpgcharacters.wordpress.com/maps/. He has adventures too - something like Challenge of the Frog Idol would convert easy to 5e (has Classic D&D stats) & work great with Primeval Thule - and if you like his stuff then Dyson's Delves I & II are available for purchase.
Alternately you could use a WoTC Adventure Path as your basis, but keep it as sandboxy as possible. I think Princes of the Apocalypse might be the easiest for this but I'm not familiar enough with them to say for sure.
The main trick is to present a living world and give the players plenty of freedom of action. A good campaign setting book like the 5e Primeval Thule one supports this; APs rarely do.
Read more: http://www.enworld.org/forum/showthread.php?484731-Thoughts-on-New-Campaign-for-New-Group-for-Returning-DM#ixzz47DPV7YKT