If you want to go down the chemical route then use a Glyphosate spray, such as Roundup. However the soil will be full of weed seeds that will immediately start to sprout once you remove the competition from the existing weeds - remember the old saying: "One year seed, Seven years Weed!"

If you prefer to be Organic and can wait a little longer then simply smother the whole area with thick layers of old carpet (readily available from your nearest recycling plant!) or black plastic - which will "cook" and kill many seeds as well as the weeds.

Are you looking for some 'instant' colour just for this summer, or planting long term perennials or biennials and shrubs?
If it's the first, the easiest thing is just to cut down all the weeds, put on a mulch of compost, for cosmetic reasons, and plant a lot of annuals very close together through that. Although some weeds will push through, the annuals will crowd out the competition and give you a good display for this year.
For a flower garden with perennials and herbaceous plants, you really MUST destroy all the perennial and deep rooted weeds like bindweed, docks, dandelions and so on. The only way to do that is to either dig over every square foot and carefully remove every piece of root, or spray thoroughly with Roundup, probably twice, with a gap of a couple of weeks.
Although as Ilex says weed seeds will come up afterwards, they will be easy to remove amongst the plants, but perennial weed roots are almost impossible to remove.
I'd recommend due to the lateness of the season now, that you plant annuals, and then dig over the area thoroughly in the autumn. You could then be planting perennials and shrubs at that time for flowering next summer.
Although I don't like chemicals, and Roundup is not as safe as claimed, many organic gardeners are prepared to use it as a one off use for initial clearing of ground.... a bit of 'God make me chaste, but not yet.'.