I've run a lot of D&D online with various tools and various editions of the game. Much of the game is similar to offline, but the lack of physicality may require certain adaptions. You may face the following challenges:

1. __Adapting to digital tools.__ It's going to take a certain amount of time before you can master the digital tools. Roll20 in particular is popular and works reliably, but has a certain learning curve on the DM's side as you need to learn the editing system. I suggest starting as soon as possible to learn the system.
2. __Preparing "miniatures" for your digital gametable.__ Roll20 and various other tools provide a good miniature simulation, which you can use as a good digital replacement for real maps and miniatures. However, regardless of your map software, you need to find or create your "minis" in advance. This isn't too difficult, but you don't really want to pause a game session to open Photoshop and create a new mini. Digital minis are also referred to as "tokens" or "pogs", and you can find some at short notice at [IMarvinTPA's monster database](https://www.imarvintpa.com/DnDLive/FindMonster.php). In my opinion you ideally want the circular sort, rather than any kind of top-down view representation of a physical mini, as these are hard to identify and you may waste time adjusting facing which is normally irrelevant in D&D 5e and Pathfinder 1e. You might also use a program called [TokenTool](https://www.rptools.net/toolbox/token-tool/) to create minis. Be sure to keep your minis well-organized so that you can find them when you need one.
3. __Draw digital maps in advance.__ In my experience, drawing with a physical pen or pencil can be fast and straightforward, whereas digital tables require you to use the various drawing tools (line, rectangle, etc), and this may be cumbersome in a hurry if you aren't very familiar with them. Where possible, you should create the maps in advance. With many tools you can uncover the parts as the players explore. Again, this is a function of the tool you will need to adapt to. Depending on the tool and how easy you find it to use, you may be able to draw maps on the fly as normal.
4. __Recruit more players when starting new online campaigns.__ In my experience, in-person D&D games have a stronger sense of responsibility to attend than online ones. My online players often dropped out, skipped sessions, or played video games in between their combat turns. As such, when starting a new game with online people, I usually recruit more players (say, 6) on the assumption that at least one will quit early in the campaign. If you're just converting an existing regular in-person game to online play, your existing complement of players may be fine.
5. __Have a policy for missing players.__ Players may be missing from a session due to technical issues or (depending on how dedicated your players are) a reduced sense of commitment. You should have a rule for what happens in this situation. Does someone play their character? Do they "fade out" and come back when the player returns? Do they get XP for missed encounters or sessions?
6. __Have a solution for character sheets, etc.__ In-person, you can pass a character sheet to the DM to check, pass a book to another player, and so on. You will need some alternative, such as digital character sheets (or just have the player photograph their sheet), and making sure players have ready access to sites with the game rules such as the D&D 5e basic rules, Pathfinder SRD or the official PDF sales sites.

A lot of what you do in-person will still be relevant. You can still invent monster stats on the fly, adjudicate combat encounters as normal, draw maps on the fly (many use fancy graphical map sheets but this isn't necessary), and describe things as you would normally.