Whenever you buy any sort of modeling kit, the model will always come disasemnbleded in whats called a sprue. A sprue is basically yhe plastic skeleton of the mini when its printed so it is easy to ship. Noe to someone who is new to building miniatures, these sprues can be really intimidating. I rember as a kid the first time I opened a box of Warhammer Space Marines and was shocked to find the box was full of scary plastic sheets. But once you learn what exactly your looking at, you won't be afraid of sprues.
When you first open your mini box you'll notice that you have multiple sprues to identify. Now if your building a gunpla than this will be easy as they color code there sprues and pu big letters on them to identify part. But for most other mini brands, you'll have to look carfully. In the case of warhammer, there sprues are usually section off based on what your building. So let's say you have a giant starter box with two whole armys. Each sprue will be dedicated to a specfic unit such a space marine intercesors on one sprue and then terminators on another. Once you understand that, you can look through your sprues and sort them based on what you need.
Now that you understand what sprues are, heres how you use them. Take your cutting pliars and find the part you need in your instruction with the number code given. Then you'll find where that part end and the sprue connector begins. Then you'll cut the part off of the sprue with the flat end of the pliars facing the part. Then rinse and repeat for the rest of the connectors are cut the the part is free from its plastic prison!