Before you become fashionable you have to decide what being fashionable is to you. You have to find your personal tastes and style. Do you like clothes that are preppy? punk? feminine? masculine? loose and messy? sophisticated and clean? etc. What kinds of colors and patterns do you like? Light? Dark? Pastel? Neon? floral? Geometric? etc . Find clothes that fit YOU and your style. Also on that note, where clothes that fit. Don't try to where clothes that are too big or too small for you. After you figure out what you like and don't like, which may and probably will be a combination of things, its just a matter of mixing and matching different things and figuring out what works for you :)
-Colors don't always have to match each other, you can COORDINATE colors too.
-Colors opposite from each other from the color wheel, (like in art class), are complimentary, meaning they make each other stand out. For example orange and blue are complimentary, if you accent a blue heavy outfit with something orange, the orange will stand out more and appear brighter.
-Colors close together on the color wheel will always create a pleasing color scheme, ex. greens blues and purples are all next to each other on the wheel so they will also look good together.
-Neutral colors, white, black, greys, and browns, go well with any other color. (note: browns/tans can be warmer or colder depending on the shade and may look better with certain colors)
-Try not to mix too many different patterns together
-Try not to mix similar but not exactly the same pattern together. ex: two pleads that are similar in color and stripes but aren't exactly the same. At a glance the brain will sense something unusual between the patterns but it will be too subtle to tell right away creating an uncomfortable feeling for the eyes.
-If you do mix different patterns together be BOLD about it
-Try experimenting with layering peices together to create more dynamic/ interesting textures/looks.
-Experiment with different accessories, jewelry, bags, purses, shoes, hats etc. they can also add another layer of interest
-Have a fashion idols, or people you look up to when it comes to their sense of style or fashion
-Try to have an overall theme/feel for each outfit you create, coordinate pieces accordingly
-Take RISKS. fashion wouldn't be where it is today if people never took any risks.
-If your going to take a risk, make it BIG. Dont kind of take the risk, take it! Trust me it your taking a risk and trying out something crazy so it might not look good in the end but it will never look very good if you only go halfway.
These are just some tips to help you get started. Remember that these are just general rules/guidelines and you dont always have to follow them. Sometimes a bunch of colors that dont follow any rules look great together, sometimes it looks really cool when people overload on many different prints, it's really all up to you how you want to dress, and what you think is fashionable. Fashion is an art, and like all arts people have different opinions about what they think is good or not, what matters is that you think its good, because it's your style, and your life. Goodluck ^ ^, hope this helps :)