Historical introduction: understanding its conjugation
The future was originally form by adding the forms of "haber" to an infinitive (remember that the h is not pronounced in Spanish):
comer he (eat I must) → comeré
comer has (eat you must) → comerás
comer ha (eat he/she must) → comerá
comer hemos (eat we must) → comeremos
comer habéis (eat you must) → comeréis
comer hán (eat they must) → comerán
In other words, the endings are identical to the present indicative of "haber". Possibly for this reason, it is by one of the most regular tenses in Spanish: there are about 12 verbs with irregular futures, plus other verbs that were created by adding a prefix to them, like decir and predecir, or poner and componer. These prefixed verbs always follow the same pattern.
This "eat you must" is still reflected in commands, like the ones in the Bible:
No matarás = you shall not kill
No irás a ninguna parte = You are not going anywhere
