Pathchy Grass - How to treat, step by step?

Hi,

Apologies in advance if this has been discusses somewhere elese on the forum (I'm sure it has as I'd imagine it is a common problem).

I've recently moved in to a property and the lawn front and back is very patchy. (only last night I cut the grass front and back).

I know diddly squat when it comes to gardening so was looking for basic step by step advice in relation to what I can do to improve the quality of my lawn?

Thanks
Mark.