There are many different brands of violin strings. Each is made of different materials, has a different thickness, and has its own special tone. The main types of strings are metal strings, synthetic core strings, and gut strings.
Most people think of cat gut when you mention violin strings. Actually, gut core strings are less commonly used today, because it is hard to keep in tune. Gut strings also tend to be pretty expensive, because it has the best tone. It is used widely by professionals. Metal (steel core) strings are known to have good volume and a simple tone. They stay in tune rather well and are easiest to play on. Non-classical professional cello for sale use these strings because of their direct sound, but classical players tend to shy away from them because of their thin sound. Metal strings are a good option for beginners, because they are usually cheaper than the other two types. Synthetic core strings are usually made out of perlon (a type of nylon). They have a rich sound that is nicer than steel core strings. These strings stay in tune rather well, but do take a day to stretch out. Synthetic strings are usually the best choice for an intermediate student.
As much as possible, try to purchase your violin strings a couple sets in advance just in case of emergency. This will keep you from being stranded before a big concert.
To put the strings on, you must take off the old strings. However, you do not want to take all of the strings off at once. The strings put around sixty pounds of pressure on the body of the violin, and the release of all that pressure at once can cause damage to the instrument. It is best to start with the E string and change them all one by one.
Twist the peg counterclockwise until the string is unwound. Pull it to take it out of the peg, and gently remove the other end from the tailpiece. Insert the head of the new string into the peg and twist clockwise. Stop after a few twists and insert the ball or loop end into the tailpiece. If you have fine tuners make sure they are about ninety percent loosened. This will give you room to go either way later. Use your other hand to keep up the slack until the string is tight enough that it does not slip out of the tailpiece (let it go when your finger does not fit comfortably between the fingerboard and the string). Continue to tune like normal, pressing in on the peg while you twist. You may also use fine tuners if you have them.
If you have problems getting the ball end onto the tailpiece, use a pencil to work it in. The string may also have a little rubber piece that slides along the string. This is to go over the bridge. If you have a string with a ball end and you need a loop, you can use your fingernails and knock the ball out. If the problem occurs in reverse, attach the loop to the side of the fine tuner.