Need help on wordpress CSS style sheet

Hi, I need help on amending the style sheet of this theme. I have made some color changes on the font color and the background color, but still can’t figure out a few things as I don’t really know html or css coding.

See below for details:

1) The blog title text and the blog tagline (at the top right and top left) to be inside the Black header area, centered and both in color code white (#ffffff).

2) The nav text “home, about, test page 1, etc” to be lower, at the bottom of the header area, so that it will not block the blog title when shift inside. Nav block also needs to be centered.

3) Post title to be in black “#000000” color.

4) The sidebar is currently floated right which I do not want. Need it to be aligned properly inside the body, with the right side margin spacing inline with that of the black header. And also, the space/ margin between the search box and the sidebar top border to be smaller bit, not so wide.

my site here: https://raintreeacademy.com/

Take a look..

That’s all! Pls help me on this simple CSS task..

Below is the top portion of the stylesheet I belived should be where the editing be.. Pls help me… ??

*/

body {
font-size: 67.5%; /* Resets 1em to 10px */
font-family: ‘Lucida Grande’, Verdana, Arial, Sans-Serif;
color:#333333;
background: #efefef;
}

* {
margin: 0px;
}

li, ul {
list-style:inherit;

}

a {
color:#000000;
}

h1 a {
text-decoration:none;
color:#000000;
padding-left:10px;
margin:0px;
color:#000000;
font-family: “Tahoma”, Helvetica, sans-serif;
font-variant: small-caps;
font-size:large;
position:absolute;
top:19px;
}

h2 {
font-size:large;
}

h2 a {
text-decoration:none;
font-family:Verdana, Arial, Helvetica, sans-serif;
font-size:medium;
letter-spacing:1.5px;
color:#808080;
}

h3 {
padding: 0;
margin: 30px 0 0;
}

h3.comments {
padding: 0;
margin: 40px auto 20px ;
}

.description {
text-align:right;
padding-bottom:5px;
}

#page {
text-align: left;
width:800px;
margin: 0 auto;
background: #efefef url(images/header_bg.gif) repeat-y;
background-position:center;
}

#sidebar {
float: right;
width:150px;
padding-right:35px;
padding-top:35px;
background: #3CB371;
}

#searchsubmit {
width:30px;
}

#sidebar p,a {
border:none;
}

#header {
width:650px;
height:100px;
padding-left:67px;
padding-top:25px;
margin-bottom:40px;

}

#headerimg {
margin: 7px 9px 0;
height: 70px;
width: 650px;
background:#000000;
background-position:center;
}

hr {
display:none;
}

/*my6*/

#nav {
list-style: none;
margin: 0px;
position: absolute;
top:70px;
left:25%;
width:500px;
}
#nav li {
float: left;
margin-left: 5px;
}
#nav a {
color: #ffffff;
text-decoration: underline;
background: #000000;
padding: 3px 3px;
font: bolder 12px/100% Tahoma, Helvetica, sans-serif;
display: block;

}
#nav a:visited {
color: #ffffff;
text-decoration: underline;
}
#nav a:hover {
color:#FFFFFF;
text-decoration: underline;
}