Two ways to point at the future, and they're not interchangeable
Spanish has two everyday ways to talk about something that hasn't happened yet. Voy a llamar a mi hermana esta noche ("I'm going to call my sister tonight") uses the periphrasis ir + a + infinitive. Llamaré a mi hermana esta noche ("I will call my sister tonight") uses the simple future tense, built by adding endings straight onto the infinitive. Plenty of English-language grammar guides present these as a casual-versus-formal pair, the Spanish equivalent of "I'm gonna call" against "I will call." That's close, but it understates what's actually going on.
The Real Academia Española's own description of the periphrasis is more precise: ir a + infinitive expresses posterioridad (futurity) and constantly competes with the future indicative and the conditional to do that job, but the periphrasis shows up far more often in spoken language than in writing, and some formal registers avoid it as inappropriate. That's a statement about frequency and register, not about which form is "more correct." Both are standard Spanish, and (as the sections below cover) the simple future can express a conjecture about the present, as well as orders or warnings that may concern later events.
Building the periphrastic future
The formation is mechanical: conjugate ir in the present tense (voy, vas, va, vamos, vais, van), add a, then add any infinitive unchanged. Va a llover. (It's going to rain.) Vamos a comer. (We're going to eat.) Nothing about the main verb changes; all the conjugating happens on ir.
When the main verb takes a reflexive or object pronoun, Spanish allows it in two positions: attached to the end of the infinitive, or moved in front of the conjugated ir. Voy a levantarme temprano and Me voy a levantar temprano both mean "I'm going to get up early," with no difference in meaning. The placement patterns for reflexive pronouns also include attaching the pronoun to an infinitive; it isn't specific to the future.
Building the simple future: one set of endings for every verb
The simple future is unusual among Spanish tenses because it doesn't split into three separate ending patterns for -ar, -er, and -ir verbs. Every regular verb, regardless of conjugation class, takes the exact same six endings (-é, -ás, -á, -emos, -éis, -án) attached to the full infinitive, not a shortened stem: hablar → hablaré, comer → comeré, vivir → viviré. Compare that to the present or preterite, where -ar verbs and -er/-ir verbs use noticeably different endings, and the future's uniformity stands out.
There's a historical reason the endings look less like typical verb endings and more like a separate word tacked on. That's covered further down, after the irregular verbs.
The twelve verbs that don't just tack endings onto the infinitive
Twelve common verbs keep the same six future endings but attach them to a shortened or altered stem instead of the full infinitive. They fall into three patterns. Five drop the infinitive's final vowel: caber → cabr-, haber → habr-, poder → podr-, querer → querr-, saber → sabr-. Five drop the vowel and insert a -d-: poner → pondr-, salir → saldr-, tener → tendr-, valer → valdr-, venir → vendr-. Two don't fit either pattern and just have to be memorized: decir → dir-, hacer → har-.
Once the stem is settled, the endings are identical to any regular verb's: tener becomes tendré, tendrás, tendrá, tendremos, tendréis, tendrán. Ser, otherwise one of Spanish's most irregular verbs, isn't on this list at all; its future tense conjugates entirely regularly: seré, serás, será, seremos, seréis, serán.
The simple future used to be a periphrasis too
The reason future endings look bolted onto the infinitive rather than woven into it is that, historically, they were a separate word. The construction developed by combining the infinitive with the present tense of habere ("to have"): cantare habeo, literally "I have to sing," gradually lost its sense of obligation and became a plain future, "I will sing." As the construction fused into a single word, cantar he became cantaré. Before the fusion was complete, Spanish speakers could still insert a pronoun between the two pieces, as in cantar lo he ("I will sing it"), a possibility that disappeared once cantar he solidified into one word.
The same process is playing out again, in real time, with ir a + infinitive. Spanish built its current simple future by grammaticalizing a periphrasis centuries ago, and today's periphrastic future is doing the equivalent job while it slowly displaces the simple future in everyday speech, a cycle documented in RAE's own account of futuros sintéticos y analíticos.
The real trap: future tense that isn't about the future
A simple future can express a present conjecture or convey an order, warning or threat. Such readings require attention to context; the directive uses can still concern later actions. The first is the futuro de conjetura: using the future tense to guess about something happening right now. Asked "¿Dónde está Juan?", a Spanish speaker might answer "Estará en casa": not "He will be at home," but "He's probably at home." Asked the time with no clock in sight, "Serán las tres" means "It's probably three," not any statement about a future three o'clock. RAE gives its own example of the same value: "Ahora mismo serán las ocho" ("It's probably eight right now"). Italian's future has the same job, which is why sarà often isn't about the future at all.
The second is modal: a future-tense statement addressed to someone else can function as an order, a warning, or a threat as well as referring to a later event. RAE's own examples cover all three: "Comprarás el más barato" ("You will buy the cheapest one," functioning as an instruction), "Te caerás" ("You'll fall," functioning as a warning), and "Me las pagarás" ("You'll pay for this," functioning as a threat). These are pragmatic uses of a future form, and their content can still concern a later event. English "You will buy the cheapest one" and "You'll fall" can likewise convey an instruction or warning.
So which one do you actually reach for?
For a plan someone has already decided on and is about to act on, ir a + infinitive is common in spoken Spanish. RAE notes that the synthetic (simple) future tends to be replaced by the periphrastic future in American Spanish, while remaining common with the conjecture value and in formal registers such as literary, judicial, and administrative Spanish. Voy a estudiar medicina can describe a plan; Estudiaré medicina can express a plan, promise or prediction; the context and variety of Spanish affect the choice.
The simple future can express a promise, prediction, formal statement or conjecture about the present. Ir a + infinitive also expresses future events and is common in conversation. Choose between them by considering the intended meaning and local usage, rather than treating one as the correct form for every plan.