hiya - if moss has covered more that 70% or your lawn, I would recommend a reseeding... kill everything on the lawn, rotovate, introduce some quality manure fertiliser in the soil, ensure good level of top soil, & seed with grass again... BUT few things:
- Might be issue with drainage in your garden - hence your moss growth - you get much sun in there? Is the ground "damp"? or has the moss just taken hold over the seasons?
- Now is NOT the time to seed.. you should wait till late summer before seeding, but over the summer you need to get the ground ready for seeding..
- You can buy moss killer - this will kill the moss, but you must rake it out once it's dead (turns black), then you can reseed the bare patches, good quality grass seed should be used.
- Also, you can scarify the garden - which will make it look very "torn" up - but also lets air, light back onto the soil again, so can patch seed on those areas...
All depends on the coverage of moss to grass & how much work you want to do...