You will need to remove the steering wheel to access the blinker/dip switch mechanism.
Once it is off, you can see what it is you need to address. The actual lever for blink/ dip function is retained by a small philips head screw & it's possible that this has come out & lodged in a place that shorts out the horn contact.

If it is the whole switch assembly that has come loose on the column, then there is a larger philips head screw that hides beneath the upper part of the blinker mechanism. Just pull the blinker arm to the 'right turn' position to access it.
This screw pulls on a wedge shaped piece to clamp the whole switch housing onto the column. Simple but effective.
See pages 171 & 172 of the Gregories 1000/1200 manual for full details with pictures.

Once you have got your car going again, hop onto e-bay & buy yourself a Gregories 'Datsun 1000/ 1200' workshop manual. They are cheap & almost always available, but will be the best thing you ever buy for your wonderfull Datsun