Mili, my hub says potatoes should be ready once the greenery has died back, so probably quite soon. We are novices at this too though!
I hope someone else can advise on garlic & onions.

Similar for them too. The leaves want to be at least turning yellow or dying back. It's a common practice to bend the tops of onions over to kink the "neck" in late summer to bulbs that have reached the required size. The leaves then die off slowly. Clean the onions and garlic thoroughly when you lift them and dry onions upside down with their roots in the air on a sunny windowsill (garage, shed or spare room) for a few weeks.

Early potatoes are ready 13 weeks after planting, main crop 17 weeks after planting. Only lift as many early potatoes as you need for a meal or two as they don't keep as well as main crop and taste better freshly dug. When it comes to harvesting your main crop try and lift them on a sunny day and leave lying on the soil to toughen the skin for a few hours so this helps to prevent damaged during storage. Store somewhere cool and dry in hessian or paper sacks don't store in plastic!

Onions are usually ready when the neck of the stem (the bit near the bulb) bends over and the tops start to die back, the easiest thing is to pull one up and if it is a decent size then they are ready. Again harvest on a sunny day and leave on the soil to dry the skin off. The onions that will keep best are those where the neck has closed up tightly ones with an open neck should be used first as they wont keep well.

Garlic is pretty much the same as onions though you don't need to leave lying on the ground to dry off unless the bulbs are very wet.