Either:
Dig it all carefully, and remove every root your see. This is a good approach, but is daunting for most newbies (and, of course an experienced gardener will know which roots are "trouble" and which are not)
or:
Wait until its growing well, Glyphosate, leave alone for a couple of weeks, then rotavate / dig / etc.
That delay may mean that you don't get some of the very early crops in ... BUT ... they will be things like early spuds that will run the risk of having their tops killed by Frost, and will be on your table perhaps 2 weeks earlier than "normal" crops. Its not a lot to forgo, in your first year, in my opinion.
Beware that some pernicious weeds are quite slow to make an appearance - Bindweed for example, so your application of Glyphosate at the earliest sensible opportunity may well not kill of some lurking monsters.
For a new allotment I think that covering with weed suppressing membrane and planting through it is a good idea. (You could use cardboard instead of buying "membrane", but do NOT use Carpet)
Do whatever you can (e.g. Glyphosate in March - mulch with plenty of manure) and then cover, and plant through that cover material.
Squash is a great crop for this scenario - relatively few plants required, quiet a long way apart (they sprawl over a large area). You can make a planting hole, through the membrane, incorporate manure etc., in the planting hole and then plant-a-plant. After one year of being covered with membrane most weeds will have given up the ghost.
But you won't have a full range of Veg. in your first year.
You can do some-and-some. Carefully dig, and remove weed roots, from whatever area you can and then, come March, move to chemical treatment for the remainder. Plant the first area with Veg plants "as normal", and use Squash etc. for the chemical-patch.
For open soil cultivation Potatoes are regarded as a good crop for "clearing a new plot". Their foliage is dense, which suppresses weeds, and the act of "earthing up" kills off any weed seedlings that attempt to grow.