How do I create a short signature? (e.g. less than 100 bytes)
The usual recommendation is ECDSA, or if you need a really short signature, BLS.  See “Security.SE: What asymetric scheme provides the shortest signature, while being secure?”, “Security.SE: How to encrypt a short string to a short ciphertext using an asymmetric encryption?”, and “Crypto.SE: Short length asymmetric encryption?” for details.

ECDSA should give you a 320-bit signature with approximately 80-bit security.  BLS can get this down to about 160 bits for the signature.  In other words, with ECDSA, you're down to about 40 bytes for the signature; BLS gets you down to 20 bytes.

If even that is not short enough, there are other schemes in the research literature that purport to offer even shorter signatures (e.g., SFLASH, Quartz), though I've heard some cryptographers express doubts about their security, so using them would be really pushing it, bleeding-edge stuff that might get broken.