I agree with the others, but there are plus sides to this unkemptness
too; the fact that its been allowed to grow long probably means there
isn't any moss to get rid of each spring and autumn, and you might have
some lovely wild flowers popping up.

A really good book for novices on lawns is The Lawn Expert by Dr
Hesayon - can get them cheap on Ebay / Amazon too. General rules are
cut it on the highest setting of the mower and often - dont be tempted
to cut it very short to save you mowing again for a while as the moss
will get hold and a few days without rain will kill off the roots of
the grass then you will be starting again!. And dont walk on it when
its very wet.

If you're going to the garden centre for some food/fertiliser, there is
different stuff for spring and autumn, so check before you buy.