The appearance of a character is important, but remember as a writer you are describing the appearance and much will be left to the readers' imagination. Of course, if you are writing for film or television or for a visual work like a comic book, then appearance becomes more important.
You should decide the physical attributes of your character. At the least you should consider:
- Height - are they tall, short, average?
- Weight - are they overweight, underweight, average?
- Skin tone and freckles, hair and eye colour
- Distinguishing features - birthmarks, scars, tattoos
- Hair color- brunnete, blonde?
- Hair length-short, long shoulder length?
Some of these attributes will be worked into the story early on to allow the reader to form an image of the character in their "minds eye". Others will be returned to as important plot devices. You should try to avoid the stereotypes - not all pirates have only one eye and have a false leg!
For aliens, physical attributes must be more detailed as the reader needs more help to picture the creature.
Think about the things your character carries and uses and whether any should be distinctive. Think of Doctor Who's sonic screwdriver, James Bond's Walther PPK, the Crocodile in Peter Pan's clock! These are all iconic accessories. People in real life tend to favour certain items and these items are part of how we recognise them and think of them. The glasses they wear, the type of watch they use, the jewellery they wear. Add accessories to shape your character. Are they fascinated with antiques? Give them a pocket watch instead of a wristwatch... Do they hanker after the past? Give them an old car like Morse had...
Clothing is usually more a plot point than part of characterisation. Your character will have types of clothing they wear more frequently - smart or causal for example. However, they are likely to change their clothing during the story based on the requirements of the plot. A good example of when clothing is a strong part of character is Arthur Dent in The Hitchhiker's Guide to the Galaxy where he wears a dressing gown and pyjamas throughout the story. The difficulty with clothing is that it can very easily create a caricature.