Okay. So, you have decided on your fabric. It has come either folded in half from being on a bolt, or 60 inches wide on a roll.
You find the pattern that you want. Hopefully is is beginner friendly. Start with a pattern that is created by someone with design experience or a degree in pattern design. I have purchased patterns from people the have design software but no experience in the design industry. They may have a degree in something totally unrelated. This is not a good idea, especially for a beginner. Do some research about the designer. That’s not hard to do in today’s world.
You need to take notice of which “view” you want. For example, this “indy” pattern which is a terrific pattern by a reputable pattern designer shows 2 different views of a garment. One is shorter than the other, and looks a little different. You need to check which view you want, and follow the instructions there. This will be similar on patterns created by “The Big Three”. When you hear this, it means Simplicity, McCall’s, and Vogue. Butterick came along a little later, but can be considered the same.
On the back of the pattern you will choose the size that you want. Sometimes the measurements can be dicey. I am just putting that out there because you can measure yourself and make the garment according to the size specified on the back for those measurements and still not have it fit. That has happened to me more than once. If you don’t want to wreck your good fabric I suggest a muslin mock up. That simply means making the garment out of an inexpensive piece of fabric first so that you are sure that it will fit. Many of us will ignore this piece of advice, just like we won’t knit a guage swatch for a sweater.
The pattern below shows that you are to measure the hip size. That’s because this pattern is for a skirt. Depending on what you make there will be more options for measurements. 
Remember it is safest to make a mock up. However, this can be time consuming. I usually just try and estimate what I think that i will be and I don’t usually sew things for myself where perfect fit is an issue. That sounds strange and really bad, but I can live with a summer shirt that is a button down that is slightly too large. I can’t live with it if it’s too small. So I usually estimate and choose patterns where exact fit isn’t crucial. If you want to make something where it is especially crucial, do a mock up. Okay. Next, go to the size you’ve chosen and buy the amount of fabric instructed there. There will be a place on the pattern to tell you what types of fabric are suitable. 
I know, you’re new at this. How’re you supposed to know the types of fabric? This is where you ask for help from the sales person, and you can also google different types of fabric and what they’re like. Pretty soon, you’ll be an expert. Choose your size, and then the amount of fabric for it.
Bare in mind that you may be adding different pieces of fabric for contrast, like a collar or cuffs. This will be indicated on the pattern as to how much you will need of contrasting fabric.
Take the fabric to the cutting table and get what you need.
There is also a section on patterns for the notions that you will need such as elastic, or buttons etc. Check this out as well, and buy what is needed for your particular “view”.
Next we will talk about taking it home and cutting it!
