If you've got it on your computer then you can get it into iBooks (which I assume you've installed) by sending it to yourself as an email attachment, opening the email on your iPad, and selecting and opening the attachment. You should then see an icon the top right corner of a box with an arrow coming out of it (if you don't see the icon just touch anywhere on the screen and you should see it) - touch that and you should get an 'open in iBooks' option (and 'open in' which should then show any other app that you've got that is capable of opening PDFs).
You can also get it there via your computer's iTunes :
add it to iTunes either by draggng and dropping it into iTunes or via iTunes' File > Add File to Library . It will then appear in the Books section of your iTunes library (if Books doesn't appear then you can add it by ticking it on the General tab in Edit > Preferences)
if not already done, then connect your iPad, select your Books library, select the PDF you've just added, and drag it over onto the iPad device
the PDF should be copied it the iBooks app