In all our travels, many people (particularly youth) come to us with a call to missions. They ask us to pray for them and ask our advice on how to become a missionary. Here are 6 things that I would say to someone called to be a missionary.

Missions starts now. In other words, if you're not making a difference for the Kingdom here, you won't make a difference to the Kingdom there.
Go on as many missions trips as you can until you feel God leading you to a specific place.
Read Celtic Way of Evangelism by George G. Hunter III
Find a great missions organization to plug into. You can't do it on your own. I obviously suggest the Assemblies of God, one of the best organization in the world at sending and equipping missionaries.
Start building relationship and connections in your area. You're gonna need them.
Pray, pray, pray, pray, pray.

This is not all encompassing by any means, but it will get you off to a great start.