Before you start the second layer, it’s important that you stagger each level - each course must maintain a 1/2 brick overlap to bond your project together. This means you’ll have to cut bricks to achieve a staggering effect.
To cut, lay the brick on the ground and mark a cutting line around the brick with a bolster and mash hammer. Now place the bolster in the groove and firmly strike it. You could also use an angle grinder with a masonry disc.