You have to scout.  There are no two ways about it.  Scouting is a major part in Starcraft.   It shows you what you can't see, lets you know the actions your opponent is taking, and it lets you know what to do and how to react! However, it is an art form - having both correct and incorrect ways of doing it.  Here are some scouting tips:

Use way points.  A waypoint is a spot in a list of spots which a unit or group of units will travel to in order.  Basically, if you scout with an scv, and you list 5 waypoints, it will go to spot 1, then 2, 3, 4, then 5.  To set a way point click your unit(s), hold shift and right click on a spot.  If you hold shift and right click somewhere else, it will go there after it has reached the first location.  It works on the mini-map also. This works well for scouting out large maps.  Early on in the game, take a peon and way point it around the map to find your opponent.

If you use way points after you have selected random, you may have blown your surprise advantage (If they see your peon).  They know what species you are, and now they can adjust their plan accordingly.  If you sneak your peon around so it can't be seen, you maintain your advantage.  Doing this is simple.   Bring your peon to a starting point on the map, but distance it from the center of that spot. Once it has reached that location, move it in short bursts until you see a building just in the edge of your view.  If you see minerals, turn back.  If you saw a building you now know what species your opponent is, you may know what he is trying to do, and he has no idea what species you are. Do the same with overlords.   Overlords move so slowly that if they see it, they know what race you are and where your base is - Just by seeing where your overlord has come from.

The most important part of scouting is not to focus on it!   If your main focus in the game is focusing on scouting, you will be so far behind.   You have to build, maintain your base, and scout.  Send your peon around and go back to your base - but keep your eye on the mini-map!  You can watch what your scout sees, just on a really small scale.