This is a question that I get asked all too often.
Ideally, you should be standing upright in a comfortable position. With Lectern mics, its best if you don’t wander around, and ideally keep facing forward whilst speaking. The more movement and head turning there is. The less likely you are to have clean audio.
Obviously every mic is different, and some may require being closer to your mouth than others. As a general rule, about 25-35cms (~1ft) is ideal.