validating and error checking

i have a 2d array but i am having trouble working out how to validate and error check my textbox's
they all have to contain numbers only but i cant get my head around it
any help would be appreciated.

    int day = int.Parse(textBox1.Text);
            int mon = int.Parse(textBox2.Text);
            int g = int.Parse(this.textBox3.Text);
            int h = a.GetUpperBound(0) - 1;
        
            for (int i = 0; i<=h; i++)
        {
            
        if (double.Parse (a[i,0]) == g)    
        {
            if (day >= 1 && mon == 1 || day <= 20 && mon == 1)