The easiest way to map your garden is by using a GPS device (eg a Leica), assuming you are not in an urban area. For this you need Autocad - you can download a trial version for free I think. Assuming this is out of your comfort zone, you need to go old school: measuring tapes, grid-paper, and a pencil, a line-level and a plumb bob.
Step 1: map your garden on paper. Set up a base-line with wooden stakes running EXACTLY N-S through the middle of the garden, a stake each 50m point. Hammer a nail in each stake getting even more accurately 50m apart. This is your baseline grid. Then set up a couple of grids at right angle, using TRIANGULATION (Pythagoras' theorem) so you have some EW grid stakes, again each 50m apart. You can do all this in one day with three people over 2ha but it is HARD work - you need 2 50m tapes and one 100m tape, lots of string, stakes, nails, a big hammer, and A LEVEL GARDEN WITH FEW OBSTRUCTIONS. (If this is not going to happen hire a bloke to do a land survey with a LEICA GPS device as above.) Grid set up, draw your grid points on a your grid paper (go with a 1:100 scale for 2ha), stretch four 50m tapes across four stakes around one grid square and start drawing square by square. To get your contour levels use a known point, plumb bob and level line (needs two people!) to build up a painstaking picture of your changing levels across the site (& wish you had just invested in the Leica hire for a day!).
Step 2: digitise. There are a number of ways of you can do this, but you are looking for 1) a free programme, 2) an easy to use programme, and 3) a VECTOR-based programme for your contours. Try inkscape (easy) or Google Sketchup free version (better, but more complicated).
Hope it helps. I map stuff like this a lot (archaeologist).