Best advice: Know what you want in a house and location. Know yourself. Know your needs. If there's a spouse involved, know your spouse's wants and needs.
Second best advice: Know what you can afford. That is NOT what you might be approved for. Know your comfort level. Just because a lender says you can be approved for "X" doesn't mean that you actually can afford it, or that you can pay it without negatively affecting your lifestyle.
Everything else comes later. Choosing the right Realtor: Extremely important, but that agent won't be able to do his/her job until you've figured out what you want in a house and what you can afford.
Getting pre-approved: Important, but it's important in the buying process, not in actually choosing a home. If you've worked out the second point, above, you're not going to be surprised by what a lender says.
Hope that helps.