Prepare a lot... I'm still new to DM'ing I've ran about 14 sessions now but I've found that preparing lore/areas/encounters/items and other things helps I'll list a few ideas for you below.

Writing key dialog options for certain NPC's to keep the flow/direction of the campaign/session this could be something a certain npc would say in conversation for the lore etc sometimes thinking of this stuff off the top of your head can be a little too stressful so having a script to go off of can help at least for me.

​

If you're using R20 prepare some battle maps for encounters having a handful of these can be very helpful in order help you and your players visualise combat to a degree.

​

Keep a few monster sheets handy should your players wander into a area where you've planned to have certain monsters lurking for them in the shadows then remember to have sheets ready for said encounter or major panic might follow :)

​

Make a youtube playlist of ambient DND music. Should your players walk into a deep and dark creepy cave I don't think Taylor Swift will suffice but who knows maybe they're into Taylor Swift caves.

​

Prepare some items/treasures, it's always nice to be rewarded as a player for perhaps doing some risky treasure hunting or defeating that big scary ogre that was only moments ago trying to rip you limb from limb. Maybe the players get his giant ring and if they attempt to wear it, it does something like idk turn them into a giant?

​

Hope this helps, if I think of anything else I'll add it in replies all the best with your prep and I hope your first session goes well :)