Usually I write the lyrics first, sometimes I get the lyric and tune at the same time (sky song), even rarer I get the tune first, then write the lyric - but the latter is how a lot of hit songs were written - a tune and a partial lyric that the rest of the band work on. There is no strict order, but if you can't write lyrics easily the only way to make it easier is to start, keep looking at what you've got, and try to improve it. Listening to music you don't normally bother with can be a good start- try to improve someone else's lyrics beyond recognition. This will naturally lead to over-writing. You listen to a song and write an alternative lyric. Don't worry about trying to write the verses in order - lots of times I start from the second verse, and the first verse comes last. Put the lyric away in a drawer until you can't remember what the original tune was, then write a new tune. Result - new song!