[name] と一緒にいるとドキドキする。[his name] to issyo ni iru to dokidoki suru.

ドキドキする！ Doki doki suru ! ...You should not use the words "I" and "You" as much as you can. This is Japanese way. A naked verb will do. "Doki doki" is Japanese onomatopoeia of heartbeat sound.