Let's Get Started!
1. Choose who you will dress as. Pick a character from a tv show/movie/book/etc. that you like. If you are new to this and are not very familiar with wigs, choose someone whose haircut and color is somewhat similar to yours. It is not too important whether the character you are choosing is the same gender as you are. For example, I love Doctor Who, and my hair is a dark brown and a bit past my shoulders. Clara Oswald has a similar length and color, so I would choose her.
2. Next, to create an authentic look, choose a specific outfit as an inspiration. If you are a super hero, many heroes have about one or two suits, so that will not be too hard to pick an outfit. However, television and movie characters have many different outfits, so it's best to choose one outfit. There are many websites that show where the different outfits the character wore were from. To continue my example of Clara Oswald, in the episode "Asylum of the Daleks", where she first appears, she wears a crimson dress that is a bit short, and has 3/4 sleeves. She also has a large belt with various tools on them, and voluminous curly hair. I would use this as an inspiration for my outfit.
3. Hair is a pretty big factor to distinguish who you are. It may be difficult to cosplay as Luna Lovegood if you have a brunette pixie cut. When a curling/straightening iron just isn't enough, and a dye job is too far, wigs are perfect. Wigs can be expensive though, costing $30 and up for most. Sometimes, it's just best to go with your natural hair color and cut, and carry it through. For example, with Clara Oswald, if I didn't have hair very similar to hers, I would need a dark brown, straight, little-past-shoulders wig. However, because the cut and color is similar, I would only need to straighten my hair.
4. Some costumes will require some accessories- utility belts, swords, screwdrivers, wands, etc. Some of these, like wands, are pretty simple to make. For more complicated accessories, you can choose to buy them, make one using your own materials, or make do with what you've got. For utility belts, maybe purchase a belt in a color the same as the original utility belt, and add tools of your choosing, or maybe tools that look like the originals. For swords, maybe make them out of cardboard, paint them silver or other colors.
5. Many costumes require makeup, but not in the same way. Some may use makeup to change the look of their features so that they look more like the person they are cosplaying as. Others use it to paint their face a color so that they'll look like a character. Maybe if you're dressing as an animal- if you're a tiger, you better get your stripes on. Maybe if you're the Hulk, and are forgoing a face mask, green face paint will be perfect for you. You don't need to put on face paint if the person you are going as has a natural skin tone that is different than yours
Use this guide to get started, and with your own ideas mixed in, you'll be a pro in no time.