Glyphosate is the active ingredient in several brands of weedkillers of which Roundup is probably the best known, so go to a garden centre or DIY store and check on the labels.
Weedkillers containing glyphosate need to be applied in warmish weather when the weeds are in active growth and have plenty of leaf to soak up the liquid. Treat according to the directions on the pack. You should not cover the weeds with plastic when they've been treated.
The trick with glyphosate is that you must leave the weeds until they die and turn brown before you pull them up.
If you pull them up before then the roots will still be alive and they will grow again and you'll be back where you started.
Work out how many square metres you need to treat - the containers will say how big an area they cover - that way you won't buy more than you need.
Once the liquid has dried it is harmless to children and pets etc, and it does not linger in the soil so as soon as your weeds are dead you can get on with planting your allotment.