Lonely
Lonesome, Solitary, Desolate
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of lonely? It's still lonely!
さびしい
さみしい
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn this reading yet, so here's a mnemonic to help you:
No one is as lonely as a sad bee (さび). The sad bee is sad because it has no friends and that makes is super lonely. Poor sad bee.
やっぱり一人暮らしは寂しいです。
As I thought, the single life is lonely.