Best answer
Both words mean lonesome or lonely in Japanese. Usually they should be 寂（さび）しい “sabishi-i” or 淋（さみ）しい “samishi-i.” Notice double “i” in the end of the words.
One exception is making 俳句 “Haiku;” seventeen-syllable short poem. In Haiku, you can use さびし or さみし.
Both さびし “sabishi” and さみし “samishi” are stem of adjectives. You have to conjugate them as follows:
・present tense-positive さびしい “sabishi-i”
・present tense-negative さびしくない “sabishi-kunai”
・past tense-positive さびしかった “sabishi-katta”
・past tense-negative さびしくなかった “sabishi-kunakatta”
Actually there are two kinds of adjectives in Japanese language.
1) [i] adjectives which ends with “i” such as 赤い “akai” red, 白い “shiroi” white, 新しい “atarashii” new, 古い “furui” old, 甘い “amai” sweet, 辛い “karai” salty, 高い “takai” high, 低い “hikui” low, etc.
2) [na] adjectives which consists of a noun + “na” such as 元気な “genki-na” healthy, 安全な “anzen-na” safe, 簡単な “kantan-na” simple, 幸福な “kohfuku-na” happy, 正直な “shohjiki-na” honest, etc.