I am a DM that has never played a PC before!!!
It helps to have at least played a few games, before running a game yourself. But watching videos of other people play does help.
but I am at a loss for what to do when players interrupt me during a dialog or descriptive scenario
Every group agrees upon their own style of playing. Frankly, I don't mind interuptions, and only on very rare occasions have I politely asked my players if I could please finish speaking. With dialog, I allow my players to interupt me at any time, because maybe they don't want to allow an npc to finish his sentence. That is totally fine.
With descriptions, especially of locations, it helps if you keep your descriptions brief and loaded with the most important details upfront. You can then add further details as the players ask questions about their environment. Whenever players enter a room they only want to know a few things:
-Is there any clear visible threat in the room (mention the red dragon first if there is one!)
-What is the size and shape of the room?
-Is it dark and can they see anything at all?
-What ways are there out of the room, and where are these located from their perspective?
-Are there any noteworthy details and/or furnishings?
You don't have to tell the players right away what sounds and smells they experience in the room, or the exact details of the walls and furnishings. The more you load your description with details, the more your players will forget, and the more likely you'll be interupted with questions. Instead, start with the basics, then as your players declare their actions, fill in the details. If they ask a question about the room, you are allowed to interpret this as their character looking around (which I consider a quick action at least). Bare in mind that if you're using an adventure module, they'll often include room descriptions that talk about "a door to the north", and that sort of description doesn't have a whole lot of meaning to your players, since they are not a walking compass. Instead, tell them that they see a door on the righthand side, or a fireplace straight across the room.
.... or skip a piece of my campaign that would explain more about what is actually going on
Whether this, or your next question become problems depends entirely on how you want to run your campaign. The players do not know what you have planned for them, so as far as they are concerned, they aren't skipping anything. They are simply pursuing what is of most interest to them. This may mean that in doing so they miss crucial information, to which I say, then spread out your information more. Don't make your adventure hinge on finding that one clue. And if that fails, you can always use an npc to spout some exposition to them, though in general I am not a fan of this DM-trick.
I've seen this trick abused a couple of times, where the players decide not to investigate something, so the DM declares that his npc investigates it for them, just to reveal a crucial bit of information. Don't be that DM. Be a bit smarter with distributing info across your campaign, and don't be afraid to move your content around behind the scenes.
... how do I get them back on track? How do I get the important information to them?
So how do you get them back on track once they are lost? Firstly, this depends entirely on whether there is a track at all. If it is a sandbox campaign, why do you care where they go? But if you do care a bit, you can always have them stumble upon a clue simply by moving the clue in their path. Alternatively though, you can also think about consequences if they do not pursue a crucial lead. Keep in mind though that you don't want to punish your players for going off track, and so it shouldn't feel like that to them. Instead, you can introduce interesting negative outcomes to them going off track, which should bring them right back on track in a session or two. If they don't kill the dragon, the dragon may burn a city to the ground, which the players then stumble upon. Once the players see the consequences of their choices, they will have an incentive to get back on track, and clear direction where to go.
For example, I had a sidequest for my players, where an ex-communicated monk asked the party to steal a holy relic from the local church. They declined. So, several days later the monk attempted the theft himself, got caught, and imprisoned. When the players visited the prison many sessions later to gather information on something unrelated, I had them run into the now imprisoned monk again. Thus this quest once again landed on their path in a way that did not feel contrived, and progressed the story in an interesting direction.
I try to think of the main plot and all the characters in it, as not just existing in a vacuum, waiting for the players to interact with it. The characters in the game world all have their own agenda and will continue about their business with or without the interference of the players. And sometimes I'll deliberately put the main plot on hold for a few sessions, so the players don't get tired of it, and can pursue other side activities.
I try to give the sand box feel but also am trying to maintain story progression.... what is the balance???
That is entirely up to you. Even sandbox campaigns have a narrative (usually, not always). You can have both. The trick is that with a sandbox campaign, you need to prepare several plothooks, and be ready regardless which one the players pursue.
I usually ask my players at the end of each session, where they are planning to go next, because this helps me prepare for the next session. I straight up tell my players that if they are planning to travel to a new city, to tell me, so that I can prepare accordingly. I think it's good to be this open to your players, because they understand that although they are free to go where they want within the sandbox, that the experience is ultimately more satisfying if you can prepare some content for them.
Now, sometimes even though my players are planning to set sail in the next session, they may linger at their current location for another session or two. That is also fine, because that just means I don't have to prepare as much.
