Lawn looking very sad

My lawn is looking very sad and full of weeds. I don't mind the odd weed and I am not looking for an immaculate lawn but it does seem to be failing more every year. I would like to know what I can do to just have a half decent lawn, eg, should I be feeding it, if so how often and with what? tia

 

@JWK, thanks for reply. I have not not tried a weed and feed combo, but have previously tried a lawn weed killer but doesn't seem to do much. Some parts are shaded by trees and it does get mossy but I did scarify in spring.

 

Try Evergreen 4 in 1 @Meomye, read the instructions carefully beforehand and you should see improvements within a few days. It blackened the moss in my lawn overnight, I waited a couple of weeks before scarifying again and then another 6 weeks before reseeding some bare patches.