Totally depends on the top soil - it isn't all the same. If its heavy and clayish you can add extra humus like leaf mould, spent mushroom compost, home made compost etc. Same treatment if its very light and sandy. If its medium you're in luck and could just add a general fertiliser such as Growmore. If you really need to know detail you can buy a simple soil testing kit from most good garden suppliers.
Some bought-in top soil are good and others are rubbish - check for stones and roots and remove any you find. You are aiming for a rich crumbly loam. Squeeze a handful. If it sticks firmly together its clay. If it won't hold a shape at all its probably sandy. If it makes a shape that easily falls apart you could be in luck!

Couldn't have put it better myself Sue, lol, Derek.

Praise indeed Derek, lol.