Joining walls

Could any one help me I am stuck trying to join walls together , I am able to copy and past walls but when I join the walls together there height can be wrong and getting horizontal and vertical right is a head ache . any help would be great .

Snap to Grid is your friend for this. You can press G or click the little grid icon. The number beside it is the snap size, the distance between points the object will snap. I'd recommend setting that to 100 when first setting down walls and floors as it makes it a lot easier to snap everything together. You can decrease the number or turn off snap to grid for fine-tune adjustments.

Snap to Grid also works when adjusting rotation. Very handy.