The more you write, the better you get. Set yourself little challenges like writing a lyric which tells a story. You don't have to use the final version, just experiment. Enjoy being creative just because you can!
Try setting aside a regular period every day to write. First thing in the morning is a good time. If you're not working on anything specific, just write anything for half an hour and see what comes out.
Try developing lyrics on a theme by writing out your thoughts as a list of ideas or a short story. Then re-organise them and work them into a rhyming lyric.
Try organising your thoughts by writing out each line of your lyric on a separate slip of paper. That way you can re-organise them and re-write lines very easily.
Suggesting a strong idea with just a few words can be tough. Avoid obvious things (eg 'as dry as a bone/desert'). Sometimes things don't even need to make sense to create a strong image (eg 'as dry as a funeral drum').
The old rhymes like fire/higher/pyre have pretty much been used to death. Try and avoid these as they can sound tired and unoriginal.
If you're having trouble making a line rhyme and flow, chances are it never will. Don't work on just that one line. You may have to re-work the previous line or two to make it all work.
You don't have to just rhyme the last word of every line. Often lyricists find unlikely phrases which work together, or rhyme things halfway through lines. Some lyrics don't even rhyme at all - you don't have to, you know!
Opinions are divided about notebooks. Some people obsessively note down every idea they have, others prefer to let their memory retain the best ideas and forget the less good ones. Try both approaches and see which suits you best.
Think of the word 'Seven'. When you say it you naturally emphasise the 'Sev' part. If you say it with the stress on the 'ven' it sounds wrong. Think about this when writing. If you force the singer to emphasise the wrong part of a word, it sounds naff.
If you're stuck for inspiration, have a look at things like cryptic crossword clues and newspaper headlines. A phrase from one of these can suggest a line or song title and get you going again.