I don't really think there is any foolproof way of ensuring that you conceive a baby girl. I think there are different theories that try to explain how to increase your odds of conceiving a girl. I've read about some in books but cannot remember what they said.
You could try going to the search engine "Google". Google.com or Google.ca and do a search on conceiving girls.
Also, by today's standards 38 is not too old to have a baby, but your odds increase of having a baby with Down's syndrome, also you have a greater chance of having a multiple birth because as you age, your eggs also age, and the chances of you ovualting more than one egg at time also increases. You should also do a search on conceiving over age 38 and see what comes up.