what type of blazer should i buy? as just a average sized 23 year-old girl
If by type you mean cut then I suggest going for a somewhat shorter, more fitted version of the classic blazer. This makes the staple piece more youthful and feminine. If you are going to invest in something more expensive, like this navy-colored example from ASOS, stick to a more neutral basic. I suggest grey, navy or cream as these basics are somewhat more interesting and young than black. If you want something even more fun, considering your age, consider getting trendier blazers in brighter colors or a cool pattern. Here is an example from Charlotte Russe. With more trendy pieces try to get them for cheaper as they are not as versatile as you age or as styles change.
For business casual look pair a printed blouse in a jewel tone under a neutral blazer. This combo is work appropriate without being drab. A pretty blouse with a print is much less frumpy or dated than pairing it with the usual plain button-up. Pair with a statement necklace for even more color near the face. For a fun day look, pair a trendy blazer with a cute tank or graphic tee.
Asos statement necklace here
Charlotte Russe blazer here
Tobi embellished tank here
Modcloth graphic tee here