>>586040a decent petticoat; a good quality cotton blouse; a neutral color pair of comfortable, versatile shoes (ie, mary janes); a comfortable, solid colored JSK or skirt that goes well with anything (if you strongly dislike solids, a print with a generic motif such as florals will work fine. you just want something that's very easy to coordinate in a variety of ways); a decent pair of lace topped OTK or crew socks in a neutral shade; at least one generic KC or headdress; and a bag of some sort.
you can always expand your collection of items into a variety of colors, patterns, and prints as you go, but as a newbie, this set of bare bones basic items will get you started and will be easy to incorporate new items into. your coords will probably be a little boring at first, but it's good for getting a grasp of the fundamentals before diving into more creative outfits. a lot of newbies start off with extravagant pieces, prints with unusual motifs, and complex color schemes and then get stuck with an incohesive wardrobe that's difficult to wear in the long run. don't be afraid to go simple, and give yourself some time to figure out what styles, motifs, and colours you really like in practice. think about how you'll wear something before you buy it. there's a lot of items that are cute in theory, but end up wardrobe bloat in the long run.