first: the first step in a sexual relationship kissing with an open mouth or using tongue (french kissing)
first and a half base: under the shirt, bra still on
second: one step up from first base, heavy petting and feeling up while making out,direct oral or manual stimulation of a partner's breast/nipple, up the shirt or shirtless for both partners.
shortstop: in baseball, the shortstop is the field position between second and third base. So, in fooling around, shortstop is being masturbated by or masturbating your partner, as that's between feeling up and oral sex.
third base: the last base before penetration sex, entailing contact with the main sexual regions, a handjob or a blowjob (fellatio) for guys and fingering or giving head (cunnilingus) to the girl
home base: reaching home base or fourth base is consumating the relationship by having vaginal sex, making love or fucking
"Margo finally let her boyfriend get to shortstop, but she used way too much lotion and he came on her shirt."
"We finally reached home base last night! Thankfully he had a condom on him because we were so horny we couldn't wait any longer"