What are the rules for flying in Cypher System? Cause I want to build an Iron Man type armor and I want it to fly.
The simple answer is "whatever the GM says". It sounds like what you want to create is a powerful artifact, the rules for which are described on page 106 of the Numenera corebook.
Based on what a full "Iron Man" suit entails, I'd say it's at least a level 8 item which the rules say would take many years to create if you aren't reducing the difficulty much. If you are a highly skilled nano with training/specialisation and putting effort into it, you might be able to drop crafting times down 3 levels or so to around a week - but that also doesn't include the time to find all the components and money you'd need to make it. You could rush-job it and make something highly dangerous and unstable (read: fun) and the GM might allow for a quicker turnaround.
Don't be disheartened though, this sounds like excellent fodder for a personal quest.
Since the resulting object is an artifact, it will have a depletion roll. That depletion roll should probably be based on how carefully it is made (so a rush job suit might be a 1 in d6, whilst a carefully crafted suit might be 1 in d20). As for when you roll the depletion roll, typically it's on activation - though if you are building multiple abilities into the suit (flight, lasers, missiles, repulsers, etc) then perhaps each one might fail separately and you'd roll each time you activated each ability.
As for how flight itself might work, simply medium to long-range movement in any direction seems adequate enough - looking at the movement descriptions for any flying creature in the creature list should be good enough to base it on.
The only rules I can find are on page 57 of the core, under the Tier 5 ability Flight of the "Controls Gravity" focus:
You can float and fly through the air for one hour. For each level of Effort applied, you can affect one additional creature of your size or smaller. You must touch the creature to bestow the power of flight. You direct the other creature’s movement, and while flying, it must remain within sight of you or fall. In terms of overland movement, a flying creature moves about 20 miles (32 km) per hour and is not affected by terrain. Action to initiate.
Since it's a Tier 5 power, I'd seriously reconsider adding it. BUT, it might make a cool Tier 5 (or Tier 4 with a slight shift in the details) ability for your very own focus: "Wears A Metal Shell."
With regard to the game and setting of Numenera, something which routinely and reliably travels at high speed is potentially game-breaking. As player or GM, exercise caution.
If you want to design & build an artifact (like Iron Man's suit) that mimics one or more existing abilities, check whatever rulebooks you have for examples.
Numenera only has a few examples of flight (the "Controls Gravity" and "Rides the Lightning" foci in the core rulebook, and the foci "Becomes Energy" and "Manipulates Force" in Character Options 2), and each is a Tier 5 focus ability, as +Longspeak mentioned.
In addition to those (since we ARE talking about a game based on Cypher rules), there are also:
From The Strange (corebook)
Embraces Qephilim Ancestry (p59)
Tier 6: Flight (4+ Intellect points). You can fly through the air for one hour, during which time your mythlight blazes. For each level of Effort applied, you can affect one additional creature of your size or smaller. You must touch the creature to bestow the power of flight (during which time it is outlined in the gleam of your mythlight). You direct the other creature’s movement, and while flying, it must remain within sight of you or fall. In terms of overland movement, a flying creature moves about 20 miles (32 km) per hour and is not affected by terrain.
RING OF DRAGON’S FLIGHT (p187)
Level: 1d6 + 2
Form: A green iron ring that appears like a dragon wound around the finger.
Effect: When the wearer activates the ring, dragon wings unfurl from his back, and for one minute he can fly up to long range. The ring does not confer the ability to hover or make fine adjustments while in flight.
Depletion: 1–2 in 1d20
From The Strange - In Translation (Character Options)
Build Robots (p33)
Tier 4: Robot Upgrade allows you to modify your artificial assistant with one new capability. One of these is "Flight. The robot can fly a long distance each round. It can carry you, but only for up to an hour between each of your ten-hour recovery rolls."
Projects Energy (p71)
Tier 4: Energy Flight (5 Intellect points). You create an aura of your energy that allows you to fly a short distance each round for up to ten minutes. You can’t carry other creatures with you.
Wears an Iron Suit (p89)
Tier 5: Armored Flight (5+ Intellect points). You can modulate the concussive release of energy produced by your armor so well that you can fly for ten minutes. If you apply a level of Effort, you can carry another creature of your size or smaller with you.
Soars Across the Sky (p83)
The entire focus is based on flight! Tier 1 allows you to Hover and slowly float through the air. Tier 2 increases that to an hour of Flight, at about 30 mph (48 kph), incrasing speed by spending Effort. Higher tiers increase those capabilities.
The Ring of Dragon's Flight grants limited flight, while Wears an Iron Suit runs up most of the "Iron Man" gamut of special abilities. However, if it's an established character, switching your focus might not be permissible, so an artifact is probably your best bet.