Use the Tables provided in the Dungeon Master's Guide

Chapter 3 of the Dungeon Master's Guide, in the section Creating Encounters, specifies some general purpose rules for building encounters. I highly recommend you read the whole chapter, since the advice is applicable to almost any normal scenario, but I'll copy the important bits for your benefit here.

To start with, the DMG offers a table of XP thesholds for encounter difficulty. Since you're dealing with Level 1 characters, I've copied the first row here:

What this tells you is that for the average party of 4 1st level characters, an easy encounter contains creatures worth 25xp per party member, or for 4 characters, 100xp overall. 200xp for a medium encounter, 300xp for hard, 400xp for deadly.

The XP values of the creatures you use can be pulled from their individual Statblocks. For example, a CR1 creature is worth 200xp, meaning a single CR1 creature should pose a "medium" difficulty for the average party of 1st level characters. CR1 creatures tend to vary in overall stats by a great deal (some have as many as 60 hitpoints while others only have up to 20), so you'll need to tune which creatures you choose based on what your party can handle (more on that in a moment).

The DMG also specifies to apply a multiplier to the creatures' overall XP value based on how many of them there are. This is the DMG table to describe this:

So a single creature is just taken as-is: a single CR1 creature is worth 200xp, exactly how it's listed in the Monster Manual. Two CR1 creatures would be worth 600xp ((200 + 200) * 1.5), putting them well-above the deadly threshold. This is because the Action Economy in 5e is very important: the more actions a creature (or group of creatures) get in a round of combat, the more powerful they are.

There are also guidelines for adjusting this multiplier based on the party size, but 4 characters is the standard for D&D party sizes, so no adjustment is necessary. Refer to the DMG yourself if you end up DMing for a party of 2, or for a party of 6+.

Finally, you need to consider the strength of the creatures themselves. You could just use Monster Manual entries to populate your games, but if you need to tune your encounters, or make your own enemies for the players to fight, you'll want the table from Chapter 9 of the DMG, under the subsection "Creating a Monster", which describes possible stats for created monsters. I've copied a small portion of this table here:

^{* The column for XP values was added by me, and isn't part of the DMG's original table}

Note that, for the most part, these numbers are "ballpark" numbers. If you look in the Monster Manual for CR1 creatures, you'll find no creatures that have hitpoints greater than 65, and only 5 (of about 30-40) that have hitpoints above 40. Conversely, almost half of the creatures at CR1 have an armor class higher than 13, or an average damage greater than 14 points per round.

These numbers are guidelines, not hard-and-fast rules

Some parties are going to be more capable than others. A party that keeps a paladin well-protected is going to be very difficult to kill off; encounters that normally rate as "deadly" often rate less so with a paladin in the party, especially as their higher level abilities begin to come online. Conversely, a party with no dedicated healers will often lose party members in even modest fights, since a single character going unconscious can sometimes be enough to outright kill them.

The best advice I can give is that you err on the side of safety in the first few encounters you throw at your players, and try to gauge what they are capable of. Then, you can begin to up the difficulty as you get a feel for their ability to recover from damage, their ability to beat down enemies, and so on.