DEIXANDO A CAIXA DE BUSCA FIXA NO TOPO AO ROLAR A PAGINA

Ola pessoal como posso  deixar esta minha caixa de pesquisa fixa no topo do site ao rolar a pagina para baixo

Tentei usar este css aqui mais não deu certo

/FIXAR BARRA /

	.fixar{

	    margin: auto auto;

	}

	.fixar:nth-child(1) {

	    height: 1000px;

	    background: black;

	    width: 80%

	}

	.fixar:nth-child(2) {

	    height: 30px;

	    background: black;

	    width: 100%;

	    position: fixed !important;

	    top: 10px;

	    left: 0;

	}

	 

<style type="text/css">
#caixa-b-1 {
	text-align: center;
	margin: 0;
	padding: 0px 0px;
	width: 100%;
	display: inline-block;
	border: solid 0px #D2D2D2;
	-moz-border-radius: 0px;
	-webkit-border-radius: 0px;
	-goog-ms-border-radius: 0px;
	border-radius: 0px;

}
.pesquisar-form-b-1 {
	font: normal bold 16px Arial, Tahoma, Helvetica, FreeSans, sans-serif;
	padding: 15px;
	color: #000;
	border: 1px solid #D2D2D2;
	background: #FFF;
	width: 95%;
	-moz-border-radius: 5px;
	-webkit-border-radius: 5px;
	-goog-ms-border-radius: 5px;
	border-radius: 10px;
	-webkit-box-shadow: 1px 1px 2px rgba(0,0,0,0.0) inset;
	-moz-box-shadow: 1px 1px 2px rgba(0,0,0,0.0) inset;
	box-shadow: 1px 1px 2px rgba(0,0,0,0.0) inset;
}
#button-submit {
	color: #000000;
	margin: 0 0 0 5px;
 width: 52px;
	height: 52px;
	border: 10px solid #D2D2D2;
	font: normal bold 19px Arial, Tahoma, Helvetica, FreeSans, sans-serif;
	text-shadow: 0 px 5px rgba(0,0,0,.6);
	-moz-border-radius: 5px;
	-webkit-border-radius: 5px;
	-goog-ms-border-radius: 5px;
	border-radius: 2px;
 background: url("./img/lupa.png") no-repeat;
margin-left: -40px;
border-width: 0px;
width: 30px;
height: 30px;

	background: -webkit-gradient(linear, left top, left bottom, from(#FFFFFF), to(#));
	background: -moz-linear-gradient(top, #FFFFFF, #FFFFFF);
}

/FIXAR BARRA /

.fixar {
 margin: auto auto;
}

.fixar:nth-child(1) {

 height: 1000px;
 background: black;
 width: 80%
}

.fixar:nth-child(2) {

 height: 30px;
 background: black;
 width: 100%;
 position: fixed !important;
 top: 10px;
 left: 0;
}
</style>

<div class="fixar" id="caixa-b-1">
 <form action="buscar" method="get" name="input" target="_top">
 <input class="pesquisar-form-b-1" id="termo_busca" name="q" onclick="" type="text" autocomplete="true" placeholder="" x-webkit-speech="x-webkit-speech" />
 <input id="button-submit" type="submit" value=" " />
 </form>
</div>

	Desde já agradeço......