how to write an equation involving hydrochloric acid and sodium thiosulfate:
HCl+Na2S2O3

I guess the first thing you have to figure out (or look up or know) is what the products of the reaction are.

GCSE SCIENCE CHEMISTRY HIGH SCHOOL - Reaction - Rate - Sodium Thiosulfate - Thiosulphate - Hydrochloric Acid - gcsescience.com. (http://www.gcsescience.com/rc4-sodium-thiosulfate-hydrochloric.htm)

Once you've done that, simply put the reactants on the left, the products on the right, an arrow between them, and balance the chemical equation.