I started writing a book and now i want to write another what one should i write first?

It's typical of the first-time writer that there are many stories they want to tell, and they can get bored of their original storyline before it is finished. Being a new writer, you should just write for fun, whenever you have time and enjoy yourself. You will know when you have found the right plot for you to take seriously as you wont be able to stop writing and the storyline will present itself to you immediately. In the meantime write whatever you want, and feel free to leave as many unfinished stories as you like, form them you can take extracts to add to your finished novels.