Here, then, are the three most common ways of telling about future events:
Using the present tense As in English, and especially in colloquial use, the present tense can be used when discussing an upcoming event. Salimos mañana, we leave tomorrow (or, we will leave tomorrow). Te llamo esta tarde, I'm calling (or, I will call) you this afternoon.
In Spanish, the time period needs to be indicated (either directly or by the context) when using the present tense to refer to the future. The "present future" is used most often for events that are happening in the near future and that are certain or planned.
Ir a + infinitive A very common way of expressing the feature is to use the present tense of ir (to go), followed by a and the infinitive. It is the equivalent of saying "going to ..." in English and is used in basically the same way. Voy a comer, I am going to eat. Va a comprar la casa, he is going to buy the house. Vamos a salir, we are going to leave. This usage of ir a is so common that it is sometimes thought of by some speakers as the future tense, and in some areas it has all but replaced the conjugated future tense for talking about the future.
This way of expressing the future has the advantage that it is extremely easy to learn. Simply learn the present indicative tense's conjugation of ir, and you'll have it mastered.
The conjugated future tense When used to talk about the future, the conjugated future tense is the equivalent in English of saying "will" followed by the verb. Saldremos mañana, we will leave tomorrow. Comeré la hamburguesa, I will eat the hamburger. This use of the future tense is probably more common in writing than in everyday speech.