The X,Y,Z values represent a Vector, whose direction is the angle, and whose length is the velocity. However the AddForce function has another version which accepts a vector explicitly rather than having a parameter for each of the x,y & z values.

In addition, if you're applying a force which is the equivalent of a single push at an instant in time (rather than a push over a duration of time), you should use a special force mode called "Impulse".

You can apply a force in the direction of a particular gameobject by using that gameobject's transform.forward property. For example, to launch a projectile in the direction of the gameobject on which the script is placed, you could use something like this:

using UnityEngine;

public class FireTest : MonoBehaviour {

    public Transform bulletPrefab;
    public float shotPower = 80;

    void Start()
    {
        // wait 10 secs, then fire every 5 seconds
        InvokeRepeating("Fire", 10, 5);
    }

    void Fire () {

        // the starting conditions (the position and angle of the 'gun' object)
        Vector3 startPos = transform.position;
        Quaternion shotAngle = transform.rotation;

        // create the projectile object
        Transform bullet = (Transform)Instantiate(bulletPrefab, startPos, shotAngle);

        // apply the firing force
        bullet.rigidbody.AddForce(transform.forward * shotPower, ForceMode.Impulse);

    }
}

well if you have the velocity that have its magnitud, and you have an angle you can transform that in a vector this means for trigonometry that velocity*sin(angle) will give you the velocity vector in the y axis and velocity*cos(angle) will give you the velocity vector in the x axis. i'm also using the force in unity to move the objects, but now i see that the velocity would be easier for the future use, so im changin to that =P

good luck