When you learned how to do an ILS approach, they probably said to fly lower than the glideslope. Usually, this is somewhere around 2,000 feet. But it can vary. A solution could be setting the autopilot altitude to the lowest setting, but, well, you're just not supposed to fly for 10NM at 100 ft agl.
The most common solution is to view an aeronautical chart of the area. You can see the altitude, there, but it's hard and I don't recommend this. I don't even know how to do it, as I have never had a problem that needed that (I use the sim, but if you fly IRL, please look at the charts).
The solution for SIM users, is a button. On some planes, there is a button labeled "CSTR." That shows constraints. Little purple numbers will appear on your map if you have configured your approach. On every waypoint, there will be one. If it has a line under it, e.g. "
" that means you have to be 2000 feet OR ABOVE. No lower than 2000 feet. If it has none, it means you have to be below that. Usually it's the waypoint right before the runway. If it says "1700", then after you pass the one right before that and are headed for the "1700" one, then set your target altitude to 1700, and THEN turn on approach mode.