If you did not observe the fast of Ramadan in the past, then you ought to repent and ask forgiveness of Allah and make it up as soon as possible. If you cannot do so because of ill health, then you must feed the poor. It entails feeding a poor person for each of the days you have missed.

Your priority is to make up the obligatory fasts you have missed. For doing so, you need to formulate the intention. Voluntary fasting without that intention cannot count towards that. However, as some scholars have ruled, one may make double intention: Make up for the obligatory fasts and observing optional fasts.

Almighty Allah knows best.