You can provide padding to an input like this:
HTML:
<input type=text id=firstname />
CSS:
input {
width: 250px;
padding: 5px;
}
however I would also add:
input {
width: 250px;
padding: 5px;
-webkit-box-sizing: border-box; /* Safari/Chrome, other WebKit */
-moz-box-sizing: border-box; /* Firefox, other Gecko */
box-sizing: border-box; /* Opera/IE 8+ */
}
Box sizing makes the input width stay at 250px rather than increase to 260px due to the padding.