Overwhelmed by inherited garden
I have an older established flower garden that I inherited when I bought this house. It has been neglected for quite some time by the previous owners which has resulted in plants and plantings growing together and becoming intertwined making it difficult to determine which should stay and which should go. My question is how do you decide that? How much if any open ground should be left between the plantings? I have included a picture which I recently took, after clearing off years of deadfall and weeds.
I would appreciate any help or advice, as I am new to gardening and feeling very overwhelmed.