If you're just starting as a DM, don't try to adjust things in your first few encounters. Balancing encounters is something that gets a lot easier with experience, and if you get it wrong early you may just wipe the party out by overdoing it.
It also depends on how optimized the party is. If the party is full of experienced players playing very strong builds, you will need to throw a lot more at them than if it's a party of newbies or people just playing non-optimized characters.
Given all that, the best advice I can give you is for the first couple of encounters, do nothing. Treat the party of six like it's a party of four for encounter purposes. If you're using a pre-written adventure, just run what they have in it. If you're making your own encounters up, use what the Monster Manual and DMG suggest for a party of their level (as a party of four, not six).
If that turns out to be too easy for them, start ramping it up. You can do that in a few ways:
- Add more Monsters. Don't do this if there's already a lot of monsters, as encounters with 20 enemies in them tend to take a really long time.
- Change to higher Challenge Rating monsters.
- Advance the existing monsters to make them tougher. There's rules in the Monster Manual to add Hit Dice to do that (or if the monster has character levels, add levels). (If you find you don't have time to do that and need to do it on the fly, adding 20% to it's HP and increasing the result of all rolls by +2 will make a tougher monster on short notice.... but you shouldn't do this on short notice if it's at all possible to plan ahead.)
Personally I find that fights in my campaign go more smoothly if the enemy count is under eight. There was one encounter in the book that had like 30 Goblins, and it really wasn't that fun for anybody. So if I needed to make a fight harder that already has six enemies, I'm unlikely to add more and will instead make the existing ones tougher.
Even with only one enemy I'll sometimes still choose to advance it. If the party is fighting a big dragon, what other creatures can I add that both make sense and won't really skew the encounter? If the fight is three enemies on the other hand, adding a fourth can be a practical way to make it harder.