i have a gallon of pure blackberry juice and i want to try and make some wine - now what ?

I've never made wine from blackberry juice - all the recipes I've followed use actual fruit.

This is guesswork mainly but I would suggest that you dilute it 50:50 with water and make two gallons. For each gallon you'll need a kilo of sugar, a teaspoonful of citric acid or the juice of a lemon, a teaspoonful of yeast nutrient and a teaspoon of wine yeast.

Use two glass demijohns but fill each one only half full to begin with or else they will overflow. Seal with a bung with an airlock. Keep in a warm (not hot) place and top up after a few days when the fermentation is less vigorous.

After three or four weeks fermentation should have finished. Rack into a clean contained and add two crushed campden tablets or 1/4 teaspoonful of sodium metabisulphite (it's the same stuff).

Ideally add finings but if you leave it long enough it should clear. I filter my wines but it's not essential. Once clear, it can be racked again, leaving the sediment behind, and bottled. Best left for a couple of weeks but homemade wine doesn't necessarily keep well.

Personally, I'm not that keen on blackberry wine but it's a popular wine to make.

Google is your friend. 

Use google, search for a blackberry wine recipe and follow the the instructions.

The best book I have found is by Berry (appropriate), First Steps in Home Winemaking. Alas, it is hard to find. Try winwmakermag.com and click on country wines.

Look up a recipe for blackberry wine.