Pesquisar na Comunidade

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......