I have to buy new shoelaces.
Common mixed kanji-kana spelling; 靴紐 is also used but may be less common in everyday writing.
Full kanji spelling; 紐 is a less common kanji, so 靴ひも is often preferred.
Compound of 靴 (くつ, 'shoe') and ひも ('string, cord'). The reading くつひも is a straightforward combination of the two components.