Although how many spaces you use is ultimately a style choice, using one space is by far the most widely accepted and logical style. The Chicago Manual of Style (1), the AP Stylebook (2), and the Modern Language Association (3) all recommend using one space after a period at the end of a sentence. Furthermore, page designers have written in begging me to encourage people to use one space because if you send them a document with two spaces after the periods, they have to go in and take all the extra spaces out.
I know it's a hard habit to break if you were trained to use two spaces, but if you can, give one space a try.
"That" Versus "Who"
A dog who wants your love...
A dog that wants your love...
First, let's talk very briefly about people. You can use the word "that" to refer to people, but "who" is the better choice (4).
She's the girl who teaches us grammar.
He's the boy who spilled lasagna last week.
(For more information on "who" versus "that" for people, see Grammar Girl episode #24.)