Connova, as Zoe stated, visit local nurseries, not big box stores, ask them questions and look at plants they have in stock. Also, talk with neighbors who have gardens of that look nice and the way you wish yours to be.
To improve the soil, I use well composted cow manure and good quality compost --I look for compost with few twigs. Then I dig either, or both, into the soil. Seeing that you already have grass, Google "trench composting". This compost method allows one to bury clippings and improve the soil--just add water or rain/melting snow. With clippings, you can also add kitchen scraps and shredded junk mail--anything organic.
