Making homemade clothes look as good as store bought ones takes a lot of time and skill but I would first start by learning the different types of seams.
The easiest seam is a hidden seam. If you look at a pair of denim jeans on the legs of the jean you have hidden seams on the inside of the leg and face stitched seams on the outside of the leg. Hidden seams are stitched with the garment inside out.
Proper pinning is also something you need to learn.
Line up the front and back of the seam and put pins in the material to hold it in place. Rounded corners will require more pins and also that you go slower as you stitch. You want to remove the pin just before that area of the fabric goes through the needle.
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If you find that you are having problems with following the lines and making them straight you can also pin your pattern to the fabric. Patterns should be cut correctly so that it is smaller then half of the width of the foot of the sewing machine where the fabric is held down and passes through. This way the outside edge of the foot will ride along the pattern as you sew and keep your lines straight.
If you look at the area around the foot you may see a gauge with measurements on it. This gauge can also be used to offset the distance from your pattern to where you want the actual stitch to be placed.
The direction that you stitch when you begin a seam is also important. You want your material to hang out of the machine and not get bunched up in the open area between the needle and the arm base. It takes a little thought but for instance you may install a zipper on a jacket by sewing the right side from the bottom up and the left side from the top down.
Most fabric stores also teach classes especially on advanced techniques or just ask them questions. After all you are buying expensive fabric and they want you coming back and not selling your machine. They may even offer free classes for beginners. It is also a place that you may meet some friends with the same interests.
Hope this helps… Remember hidden seams are easier.. Pin the material in position before sewing, Pin the pattern to the fabric and go slow…