how to draw text around the circle in C#

It's difficult to answer without more information, primarily I'd need to know what constraints you have on the spacing.

If you're just trying to evenly space the text all around the circle then something like this might do the trick for you;

using System;
using System.Drawing;
using System.Windows.Forms;

namespace CircleText {
    public partial class Form1 : Form {
        public Form1() {
            InitializeComponent();
            SizeChanged += (s, e) => Invalidate();
        }

        protected override void OnPaint(PaintEventArgs e) {
            base.OnPaint(e);

            var center = new Point(Width/2, Height/2);
            var radius = Math.Min(Width, Height) / 3;
            var text = "ABCDEFGHIJLKMNOPQRSTUVWXYZ";

            var font = new Font(FontFamily.GenericSansSerif, 24, FontStyle.Bold);
            for (var i = 0; i < text.Length; ++i)
            {
                var c = new String(text[i], 1);

                var size = e.Graphics.MeasureString(c, font);
                var charRadius = radius + size.Height;

                var angle = (((float)i / text.Length) - 0.25) * 2 * Math.PI;

                var x = (int)(center.X + Math.Cos(angle) * charRadius);
                var y = (int)(center.Y + Math.Sin(angle) * charRadius);


                e.Graphics.TranslateTransform(x, y);

                e.Graphics.RotateTransform((float)(90 + 360 * angle / (2 * Math.PI)));
                e.Graphics.DrawString(c, font, Brushes.Red, 0, 0);

                e.Graphics.ResetTransform();


                e.Graphics.DrawArc(new Pen(Brushes.DarkGreen, 2.0f), center.X - radius, center.Y - radius, radius*2, radius*2, 0, 360);
            }
   }
    }
}

Hope this helps