Each turn is divided into phases, and most phases into steps. You can only cast a spell or activate an ability at a time that the game allows you to. Because one of the rules for the Untap Step within the Beginning Phase says you can't cast spells or activate abilities you will need to wait for the upkeep, see the rules below:
502. Untap Step
502.1. First, all phased-in permanents with phasing that the active player controls phase out, and all phased-out permanents that the active player controlled when they phased out phase in. This all happens simultaneously. This turn-based action doesn't use the stack. See rule 702.24, "Phasing."
502.2. Second, the active player determines which permanents he or she controls will untap. Then he or she untaps them all simultaneously. This turn-based action doesn't use the stack. Normally, all of a player's permanents untap, but effects can keep one or more of a player's permanents from untapping.
502.3. No player receives priority during the untap step, so no spells can be cast or resolve and no abilities can be activated or resolve. Any ability that triggers during this step will be held until the next time a player would receive priority, which is usually during the upkeep step. (See rule 503, "Upkeep Step.")
503. Upkeep Step
503.1. First, any abilities that trigger at the beginning of the upkeep step and any abilities that triggered during the turn's untap step go on the stack. (See rule 603, "Handling Triggered Abilities.")
503.2. Second, the active player gets priority. Players may cast spells and activate abilities.
As you can see by the text I bolded, the first chance for a player to cast a spell, even an instant, is during the Upkeep Step after triggers from the Untap Step and the beginning of the Upkeep Step have been placed on the stack.
If even wanted to find some back-alley way to give a player priority during the Untap Step you wouldn't because the line saying spells can't be cast and abilities can't be activated will override any other action. In the game of magic, a "can't" (can't be cast/activated) always beats out a 'can':
101.2. When a rule or effect allows or directs something to happen, and another effect states that it can't happen, the "can't" effect takes precedence.
Example: If one effect reads "You may play an additional land this turn" and another reads "You can't play land cards this turn," the effect that precludes you from playing lands wins.
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