Learn Spanish (Spain): Mañana hablaré con el jefe. - Tomorrow I will speak with the boss.

Unlike the present tense where you chop off the ending, the simple future tense is built by taking the entire dictionary form of the verb (hablar) and adding the future endings. For the 'I' form, the ending is -é, giving us hablaré. This same pattern works for almost all -ar, -er, and -ir verbs!

Is there a difference between using hablaré and voy a hablar here?

At this level, you can use them almost interchangeably. Voy a hablar (I am going to speak) is often used for plans that feel very certain or are happening right away, while hablaré (I will speak) is the standard future tense. In spoken Spanish in Spain, you will hear both used for actions happening tomorrow.

Why does hablaré have an accent mark on the last letter?

The accent mark tells you to put the spoken stress on that final letter: ah-blah-REH. In the simple future tense, the forms for 'I', 'you' (informal singular), 'he/she/it', and 'they' all carry a written accent mark on their endings to ensure the stress falls on the end of the word.

Doesn't mañana also mean 'morning'? How do I know which one it is here?

Yes, it can mean both! When used as a time word on its own, mañana means 'tomorrow', as it does in this sentence. When it means 'morning', it acts as a feminine noun and will usually have a word like 'the' or 'this' before it, such as la mañana (the morning) or esta mañana (this morning).