Jim, the guy who maintains Missionary-Blogs.com, is hosting a bloggy carnival on the topic of advice to people who want to be missionaries.
Of course the advice given will depend upon where in life you are and just how far along in the process you are to becoming a missionary. I mean, if you are an unattached 19 year old asking the question the advice I'd give you would be somewhat different than the advice I'd give to a married couple with four kids. So, since I came the unattached college student route, that's who I'll pretend I'm talking to here, and maybe some of t he others who participate in the carnival will address other demographics.
So, if you are a high school student or college student interested in becoming a career missionary, here is my advice to you in no particular order.
1. Stay out of debt. Do everything you can to avoid living beyond your means.
2. Talk to and pray for missionaries. When they come to visit your home church, talk to them. Read their blogs, leave comments, join their prayer mailing lists.
3. Go on short term mission trips. For example, I first went to Houston for 10 weeks after my junior year in high school, a month to Taiwan after my freshmen year of college, two months to another Asian country, then a year back in Taiwan.
4. Get involved with local opportunities to serve in your church or other churches in your community. Is their a local soup kitchen? Does a nearby church have a ministry to refugees? international women? inner city kids after school programs?
5. Become a mission info sponge. Find out all you can about different missionary organizations, read biographies of missionaries (And the Word Came with Power is a great one to start), go to mission fairs, take a Perspectives course. My grandmother is part of a "mission circle"--a bunch of older retired ladies meet once a month to pray for missionaries, share books read, or listen to a missionary talk about their work--I was the youngest in attendance by at least 30 years. But meeting with these women was one of the greatest blessings as I journeyed to become a missionary.
6. Make international friends. They are all around you, and every single one I met was longing for a friend. The world is at your doorstep . . . international students and business men (and their wives and children) would be happy to be your friend and teach you about their home culture.
7. Read about and pray for the lost. Use something like the Joshua Project or a book that introduces different unreached cities and people groups.
8. Surf the websites of mission organizations you are aware of to look for their advice and their requirements.
If you are a little more sure about where God is leading you, you can do all of the above with a bit more focus on the specific people group or country.
And, above all, pray. When I begin to search and ask questions and hunt for answers on how to get myself overseas . . . I became like the ever twisting cycle of a washing machine--churn, churn, churn. I think James referred to it as being tossed about by the waves. I asked so many questions and talked to so many missionaires . . . and each one gave me different answers, perspectives. I am thankful for their input, but until I realized I was trying to be a missionary on my own power I wasn't at rest.
The path may be unknown to me and to you, but it is not unknown to our Heavenly Father. Speak to Him, seek Him, and He will faithfully make known to you the path. He says so, and we both know He is trustworthy.