How do i detect when two parts are colliding?

There are a couple different ways you could go about this, and it depends on your definition of a collision.

If a collision is when two parts are intersecting you could use the GetTouchingParts method to determine which parts intersect another. You could then search the table for to see if the desired part is intersecting. Note that this does not count objects that have their CanCollide property set to false.

local part
local otherPart
local colliding = false

local function checkCollision(part, otherPart)
    for _, part in ipairs (part:GetTouchingParts()) do
        if part == otherPart then
             return true
        end
    end
    return false
end

part.Changed:connect(function()
    checkCollision(part, otherPart)
end)

otherPart.Changed:connect(function()
    colliding = checkCollision(otherPart, part)
end)

If a collision includes when two parts are touching but not intersecting (no space between faces, but not intersecting), then we can use the Touched and TouchEnded events. Be wary though, as if two parts are already touching when the game starts, the Touched events will not fire.

local part -- A part to detect touch events on
local otherPart -- The part to check for collisions
local colliding = false

part.Touched:connect(function(hit)
    if hit == otherPart then
        colliding = true
    end
end)

part.TouchEnded:connect(function(hit)
    if hit == otherPart then
        colliding = false
    end
end)

Note that neither of these methods are overly efficient, and would not rely heavily on them. If you are simply checking for collision of these two parts, go ahead. But I would not suggest using this as a custom collision detection engine (nor would I suggest making such a thing in Roblox)