You push the tip of the turn-signal stalk toward the steering column for the horn. Don't be shocked if it doesn't work.
You might consider ordering the
Bentley "shop manual" for your year MGB (actually the older model book is useful too, to an extent). <span style="font-weight: bold">The front of the manual has a reprint of the owners manual that came with the car with a detailed explanation of all the knobs and switches, etc.</span>
That book will be invaluable, trust me