public partial class Form1 : Form
    {
        string[] a = new string[50];
        int polozka = 0;

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            a[polozka] = textBox1.Text;
            polozka++;
            a[polozka] = textBox2.Text;
        }
    }
}