We have a positive electric charge (+) that is moving upward (let's call the up direction the +y direction).
This charge is entering a magnetic field that is pointing outward from the screen (let's call this the +z direction, out of the page).
We know that a magnetic force acts on a moving electric charge, and it is always perpendicular to both the direction of the charge's motion and the direction of the magnetic field.
So if the charge is moving in the +y direction, and the field is in the +z direction, the force must be either in the +x or -x direction (left vs right).
To determine which way the force pushes the charge, we use the "right hand rule":
1. Point your index finger in the direction of the charge's motion (+y up)
2. Point your middle finger in the direction of the magnetic field (+z out)
3. Your thumb will then point in the direction of the magnetic force (+x left or -x right)
In this case, following the right hand rule, my thumb is pointing to the LEFT (-x direction).
Therefore, the magnetic force on the positive charge moving up into a magnetic field pointing out of the screen will be to the LEFT (-x direction).
The step-by-step reasoning is:
1) Charge is moving up (+y)
2) Field is out of screen (+z)
3) Force must be perpendicular to motion and field, either +x or -x
4) Apply right hand rule with +y and +z
5) Thumb points LEFT (-x), so force is to the left.