Plastic and nylon mallets are a useful tool for all players. Metal mallets produce a better quality sound but require more of a deft touch.
Tenseness will wear out your arms. You’ll also strike the keys too hard.
One way to check your grip is to open up your hand. Try to balance the mallet on your index finger. The spot where the mallet feels balanced is where your grip should be.
Try striking the lower bars they’re near. Start with the left bar in the bottom row, then strike the small key in the top row, and finish with the right bar in the bottom row. You’ll hear how the tone gradually increases as move right on the glockenspiel.
The notations such as C or C4 represent notes on the musical staff. The notes are named by letter. They go A-B-C-D-E-F-G and then start again at A. The sharp notes are represented like C#. Flat notes are represented like D♭. C# and D♭ sound the same and would be found on the small key between the C and D keys. Investing in a book on how to read music is very useful for learning. These books typically have a few simple songs you can sound out while memorizing what notes to strike.
You can also try playing the individual notes of chords. The C Major chord arpeggio is C, E, G, C, E, G, C. Play the notes in order to practice your playing motion while learning the sounds of the notes.
This grip is called the traditional cross grip and is commonly used outside of America. Many American players are taught the Burton grip. The mallet shaft farthest from you crosses over the other one. Your index finger goes between the shafts. Your thumb stays on the outside. A third grip is the Stevens grip. The mallet shafts do not cross. Place one mallet between your middle and ring fingers. Use your pinky and ring fingers to hold it. For the other mallet, set the bottom of the shaft in the middle of your palm, resting it against your index finger, then pinch it between your index finger and thumb.
One chord you can try is the C major chord. Find a C key. Find the E and G keys next to it. Use one hand to reach either the C and E or E and G keys. Reach the other key with your remaining hand.