When to conceive?

I'm 24, I have a 3 yr old daughter already, and I'm going back to school and working. My fiance and I both want another baby but right now isnt the right time. I want one within the next couple years so that my daughter isnt too far in age from her sibling yet I dont know if we'll be financially ready by then. Any input from fellow moms would be great, my mother said leave it in gods hands but I want to be smart about this and do whats best for the family.