How important are looks against personality for 18 year olds?

Don't put so much emphasis on looks. Do dress and maintain yourself well though. You should be confident and comfortable in your appearance and it all should match with your personality. As far as looks go, that's all you should really worry about it because looks and beauty is subjective. Attracting women is mostly about stimulating their minds and showing yourself as having high value, confidence, and a fun personality.