Hey there, I'm a new Dm playing a party of 4 players whom I'm introducing to the game, only 2 really come out of their shell and reach out to play creatively but all 4 are still inexperienced so I started them off as all Level 1:To grasp the party we have a Tiefling Bard, A half-orc Barbarian, A Half-Elf Monk and A Human fighter (Ill be adding another human rogue and Half-elf wizard in a bit though). Now the issue I'm having is that I set up a plot for the players but have no ready boss to fill in the plot, the idea is that west to the town is a river that's common to fish at, the river cuts through a fae forest and many women and children have been disappearing (it's common for women and children to do their washing in the river or net-fish to earn a living during the day). The party eventually explores the area and come across a large tunnel, they go into it using their darkvision and at the end find a trench filled with dead animals and bodies of said 'missing' people, the people are a bit torn and bloody, with bite marks in fatal spots. Now my original plan was to use Gnolls or possibly even a Displacer Panther (Thought a displacer beast would be much to high lvl for them). Now I'm stuck looking for a monster(s) that can fill the plot position while being exciting enough to push the party without doing too much damage as once again I started them as lvl 1 characters. Please help, I'm a newer dm and im not sure what to do.

There's always the option to nerf the encounter to suit the levels involved. As long as you aren't pitting your level one players against something that's meant to be a significant threat like a vampire or a genie, I think it's entirely reasonable to just nerf a slightly higher CR monster to fit better with the level of your party. My group's DM admitted she had to do that, because she accidentally picked a premade adventure for our level one starting adventure that was intended for a party of seven... and there were four of us. Which she only realized when it took us an entire session to take out the first encounter, a mob of three scarecrows. These things happen, no harm or shame in fiddling with stats.

There are a couple of ways to go about this.  The first is like Void said and nerf a strong monster to fit the group.  Another is have them do enough to start leveling them up a bit to take on a stronger monster.  An idea that I came up with (and it'll keep the adventure going), is to have a pack of wolves be responsible for the attacks only to find evidence that they were being controlled and they now have to find out who is controlling them.  This scenario is kinda like the headless horseman myth but without the headless horseman.  It could be a druid that has some issue with the townsfolk, or a townsfolk that has issues with the town and is trying to destroy it.  The sky is the limit with what you can do and you are only limited by your imagination.  

Hope this was helpful, and as always, have fun and kill monsters.

You can take cr 1/2-1 beasts and give them an interesting core feature from a monster you really like to add that flavor without making a monster unfair for your players to deal with. Displacer beast too strong? Try a displacer wolf. You can still flavor it the same way but it would be a reasonable opponent. 

Another fun way to handle an encounter, one that I've used before, is having a bad guy who uses minions. An Ultroloth is much too strong for even a large party of level 6 characters, but if it summoned some lesser fiends to test the party, suddenly you have this still potentially lethal encounter with a really bizarre foe that *isn't* guaranteed to slaughter the party. With something like that you can even give your players a bad guy to gear up and build towards, a metric to scale to for a future rematch. It can be fun with the right group. 

Lying is a new DM best friend. If you are rolling behind a screen they can't see your numbers. You are introducing these people to the game, make them feel cool! If your players cast a cool spell but the creature passes it's save, have it fail anyway! When they hit their creature with an attack describe it in a way that makes them seem bad ass! If the creature hits, roll the dice but  just make up the damage so that it can feel dangerous but not actually be fatal. Over time your players will understand the game better and become more confident, as they do so you can take off the kiddy gloves. By 4th level you should be able to just use the numbers rolled and let your players deal with the consequences.

In shor,t lie so that your players have fun, until they actually know what they are doing. This way you can pick any monster that seems interesting and not worry about the crunch.

If the party is of 6 lv. 1 players, a Meenlock from Volo's Guide to Monsters might be a nice boss. Fey that enjoy more than anything to spread fear, and often inhabit forests or subterranean terrain. It's CR 2, so it should prove a proper boss for 6 lv. 1 players. If the problem still stands, here's the stat block http://chisaipete.github.io/bestiary/creatures/meenlock