If your public key can be static

pub const YOUR_PUBKEY: Pubkey = solana_program::pubkey!("base58_pubkey")

Otherwise

let your_pubkey = Pubkey::from_str("base58_pubkey").unwrap();

Here is an exact example (the string will vary with what you have) that converts a base58 string to Solana Pubkey:

let pk_key = Pubkey::from_str("SDMHqNqN82QSjEaEuqybmpXsjtX98YuTsX6YCdT99to").unwrap();

I've never used solana before, so I am not sure exactly how one would do this. Looking through their docs, it looks like PubKey implements the FromStr trait. This has a from_str function that takes a &str and parses it into a PubKey.