Make sure it's a sunny area. Vegetables won't thrive in heavily shaded locations.
Mark the area out with string and edging boards, and using a spade, skim off the top turf with as little soil as possible working across a row at a time. Stack the unwanted turves upside down in a corner and let them rot down, where in a year's time they will give you nice pile of compost to spread back on your patch.
Deep dig the soil as soon as it's dry enough to work, and dig in some well rotted manure and rake level. Remove as many big stones as possible to get a fine tilth. You can buy sealed bags of manune from garden centres, normally around 3 for £12.
Watch how the sun moves around your garden so that you plant tall things like tomato plants, climbing runner or French beans in a location that won't shade other lower vegetables from the sun.
Grow only vegetables you like to eat, and if space is limited, choose those which are expensive to buy, tomatoes, beans, courgettes, etc. Don't sow a whole packet of seeds at a time or you'll have a glut you won't be able to eat. i.e. if sowing lettuces, just sow half a row at a time, and another half row in 2 or 3 weeks' time. . Good luck and hope you have fun growing.