In simple terms, money in vs money out. Take a look at what you earn, and what you spend. Reducing what you spend and increasing what you earn will improve your debt to income ratio.
This ratio is looked at by lenders and each will have their own percentage that they are comfortable lending on. If you think you are struggling here, you could improve this by reducing your outgoings or even taking on extra work.
This is often simpler than you might think. A few changes like eating out less or cancelling that gym membership you never use can make a difference. It all adds up.