When you are first starting out learning the guitar, it's hard to get your fingers to do what you want them to.
That's because they need to be trained.
You must bend them to your will and teach them who's boss! But in the beginning, that seems hard to do.
You need to know that this happens to everyone.
What you are trying to get your fingers to do is not natural for them - They are just not used to being used like that.
But fingers learn quickly so don't worry.
One of the hardest things in the beginning is to get each fingered note to sound clear.
This is because your fingers are made of flesh and bone, while most strings (unless you are playing what's called a "classical guitar") are made of steel.
Steel wins - until you develop callouses (or hardness) on your fingers.
After a couple of weeks, with practice, your fingers will get these callouses.
  It's very important when you play that all of the notes are clear as a bell - No thuds allowed! Make sure that your fingers are as straight down on the string as you can make them.
If you are playing chords, you not only have to make sure that the notes that you are fingering are clear, you have to make sure that all of the open strings around them are clear too.
This is sometimes hard even for us experienced players and can be a constant battle if we aren't paying enough attention to proper technique.
So when you practice either single notes or chords, make sure that everything is clear.
Remember - No thuds allowed!