To discuss future events, you just use the normal present tense. It will be obvious from context that you’re talking about the future. Instead of saying “I will not come”, you just say “I come not”.
“Að koma” means “to come” and “ég kem” means “I come”. The K in koma is a normal K while the K in kem is a K in the roof of the mouth . The sound is pronounced by having the most backwards part of your tongue touch the roof of your mouth, and is close to the sound in the English word cute. This sound always appears if an E follows a K.