a 'mini' sewing machine might not be able to handle big projects like costumes. Your best bet is to make simple things. Maybe something like doll's clothes or a type of clothing that doesn't neccessarily require a pattern.
Generation T is a good place to start to get a feel for making clothing. I would recommend you get it from your library or bookstore. It doesn't primarily deal with sewing, but you'll be making clothes from used t-shirts.
...I don't know. If I were you I would hit the books (like I did. I had to teach myself.), or find someone who has made garments before. Because when you sew clothes, it can get confusing and complicated if you don't know what you're doing.