Step 1: Wash and dry how you would normally care for your dirty clothes or take to dry cleaners BEFORE you start the next step.
I should also mention that your first attempt at sewing clothes should be a simple design. An example would be skirt or a dress. Shirts can be challenging but only if your sewing machine doesn't have buttonhole attachment. Pants are also another challenging item and would require some extra experience.
So lets pretend you bought yourself a skirt pattern. A simple flowing skirt with an elastic waistband. See image below.
Step 2: Take out your pattern, laying out each page out on the table or floor (if you need more room). Open your pattern instructions and look for the illustration telling you which pattern pieces go with your skirt. Those pieces will be numbered for easy reference. Cut out the pieces you need and set aside.
Step 3: Iron your fabric (if the fabric can use the iron), you don't want wrinkles when trying to cut out pattern pieces.
Step 4: Now take your laundered fabric (or non-laundered if you bought dry clean only) and fold in half length-wise. The right side of the fabric should be folded in (or right-side together), with the wrong-side of fabric showing. Smooth out creases or wrinkles until the fabric is laying flat.
Step 5: Looking at your pattern pieces illustration take the pieces you cut out and lay them on the fabric as suggested. Note: Sometimes patterns have you place the pattern pieces right-side down (usually shown has a darker color), just check first with the instructions. Pay close attention to pieces requiring a folded edge and place accordingly.
Step 6: Pin your pieces to your fabric until each piece is secure.
Step 7: Cut out your pieces. Pay attention to notches ( those little triangles) and cut those out too. Notches are used to help pin fabric pieces together to make the clothing.
Step 8: You have completed the first segment on preparing the fabric and cutting out the fabric.