이나 after a consonant and 나 after a vowel.

If the last syllable ends with a final vowel you use -나. Example as written above: 여자나 남자 (because 자 ends with the vowel ㅏ).

If it ends with a final consonant you write -이나. Example: 책이나 연필 (because 책 ends with the consonant ㄱ).

The same goes for many other endings like -(으)로 meaning by/with a. Example: 핸드폰으로 and 차로.

Note that when using -(으)로 there's an exception so you'll still only use -로 if the final consonant is ㄹ. Example: 연필로.

This is my first time explaining so I'm sorry if I made it too complex to understand. I've included links to the websites I used below.

For -(이)나 talktomeinkorean.com/lessons/l6l18

For -(으)로 www.learnkoreanlp.com/2008/08/particles_30.html?m=1