D&D 5E DMing for Only 1 Player [Any Tips?]

You don't have to give them NPCs or multiple characters to have fun solo gaming.

As the DM, you just have to realize they are a party of 1 and have limited resources and secondary options available to them. Vastly cut down on the opposition in combats and give them plenty of opportunities to achieve objects without it (stealth, talking, trickery, etc.). Think of ways to "Say yes" to whatever the player comes up with.

Beware save or suck and save or die effects. With a party of one, they become party killers. If the PC drops, you have a TPK.

Also, in this same vein, don't feel that every combat needs to end in PC death if they drop to 0HP. Have the monsters capture the PC or just KO them instead unless they are ghouls or some other savage fiend.

Give out inspiration A LOT. With a single PC to focus on, you really shouldn't be slacking off.

Think of them as a hero in a book or movie or video game. Most of those characters get by just fine without a party.

I started in 4e when my brother ran me through Keep on the Shadowfell... He gave me an NPC companion (and later a second PC to control), and kept it interesting by not sticking to the book.

DMing for a single player has a lot of similarities with a group, and you will see that most of them work equally well for more than one person, but my biggest tips are:

1. Echoing [MENTION=7989]Wrathamon[/MENTION], don't be afraid to throw an artifact at them (or powerful magic item), even at a low level. You could have them come across an artifact, have the story revolve around it for a time, then move on to bigger and better things-- The plus side of which is that you can make encounters more difficult and more interesting, because they'll be buffed up beyond what they should be.

2. Don't start them at first level. Keep in mind that they'll burn through resources a lot faster than a full party, and being alone they won't have anyone to take hits for them. Personally I like levels 3 or 4 to start, because they don't breeze through the levels anymore and have the foundation of their class build done (especially path specializations).

3. I know this is a common sentiment, but give them support! Add in a second PC for them, or an NPC to help them out. It may reduce their sense of vulnerability, but ideally it should make them more invested in the world because they develop a relationship with this NPC (similar to in-party friendships).

4. Make their changes matter. In a game I DMed for a single player, he was framed by a doppelganger for assaulting an innkeeper and stealing a magical statue. I expected him to kill the doppelganger, return the statue, and claim the reward, but instead he decided to take the doppelganger under his wing. It's always a bad idea to DM fiat against what the players want in any campaign, and if there's only the one person you don't want to give the game a sense of being adversarial (DM and player playing against each other).

5. Try to avoid a high turnover rate. In my aforementioned Keep on the Shadowfell game I played, my cleric ended up petrified, so I had to go on a whole side quest to get it fixed. Because I had no gold, I enlisted the help of a medusa who tyrannically ruled a city (crippling the rebellion and kickstarting a civil war in the process) to get it fixed. If I'd had to reroll a lot of characters before that point, I likely would have taken the stance of "reroll, because it's easier than doing the side quest". Tied in with this is to have penalties on character death (-10% XP or something) so that they avoid making stupid gambles that have little or no chance of success, or a minimal reward at risk of dying.

And lastly, 6. Give a way to even the odds. Provide ways to bypass potential combat encounters (and make sure you remember to give them XP for defeating non-combat encounters), or in situations where they absolutely have to fight, make terrain and obstacles that they can use to their advantage (such as driving enemies into a pit trap or forcing them to bypass a choke point). Reward them taking a minute to scope out their surroundings and come up with a better plan than charging in screaming.

Well, that's all I can think of. DMing for a single player isn't for everyone, but it can be just as interesting as with more people if done properly.