Advice for DMing D&D Next.
Since the play test is over. I'd suggest you just go play 1E or 2E instead. Its much more balanced, tested, and overall a better game.
If you insist on playing the play test, realize that it will take a master DM that can make things up as they go, know when to allow auto-success, auto-failure, and when to make the players roll for something. There is little to no guidance except "how hard do you 'feel' this should be?" which is pretty much meaningless unless you are a very experienced DM. I'd suggest reading the skill and encounter creation articles over at
www.angrydm.com. They will help any game regardless of how badly put together it is.
The monsters are push overs. You will have to adjust. For the most part you might just want to throw out their entire hit point totals and just track how many times they get hit and determine when you want them to fall. For instance kobolds will probably fall on the first hit no matter what, whereas an Ogre is probably going to take 10+ hits to drop.
Watch out for save or die and save or suck spells and monsters, they are riddled throughout 5E. Keep in mind its no fun for a player to die and have to create a new character because they rolled low on a dice. If you use these, give plenty of warning and give a way to avoid the save. Players will use these effects often so don't get attached to a creature. If the players can see it, its fair game to kill it with a single spell or hit.
Overall again, I'd just urge you to go play something else as 5E as it stands now is really rough and not even as well put together as the early editions of D&D...
Please do not pay attention to Lokiare's advice. You want to try D&D Next, so let's help you with that, shall we?
First, low level monsters can be easy to hit and kill. But heroes have limited hit points and resources as well. Tossing 7 goblins at a group means that at least 1 PC is going to take 3 points of damage, after which the group will have to determine whether they want to take a rest to heal up, use a spell, or just keep going. PCs can feel fragile too. That can build tension for the story. I say let them get some easy fights in to get used to the combat system. Maybe one or two tougher fights towards the end, to let them feel fear or a challenge.
Secondly, the game's action resolution system is based on "ability checks" not skill checks. The game wants to train you to use a certain vocabulary for making ability checks. You're encouraged to say "Make a strength check to jump over the ravine." or "Everyone, please Make a Wisdom Perception check." This tells people what the ability check is that they are supposed to make, and educates them about any proficiencies (skills, tools) that can help with that check.
It is not hard to determine if you want a player to make an ability check. If there is a penalty for failure, or if time is of the essence, or something is particularly hard, then sure, have them make an ability check. Otherwise, if success is inevitable over time, and there is no chance for failure, then just say they succeed and tell them what happens when they succeed. If it was a hard job, say that it took them a little time to succeed.
If they search a broad location, and something is hidden there, have them make a search check. If they say they want to search a specific location and there is something to find there (coins in a creature's spoor or on its body, a coffer under a bed, a secret door at the dead end of a hallway, then just let them find it. Reward them for interacting with the environment.
Read the How to Play document and DM Guidelines document. They are indeed helpful.
Don't be a slave to the numbers. If you make a fight too easy, add a complication to the mix (wandering monster or reinforcements or something). Or just let the heroes feel awesome with their butt-kicking. If you made it too hard, fudge some rolls, or make the enemies overly bold and spend actions to gloat, or have an ally or two leave to deliver messages to leaders.
Something that is helpful is to give a reason why the bad guys don't want to kill the heroes. Give the heroes a chance to use ingenuity to escape. Things like that. Later, when you and the players have more experience and the stakes are higher, you don't need to pull as many punches.
I have to get going, but that's my first bit of advice.
... have fun!