We must not supplicate or devote acts of worship to them but to Allah alone (Glorified and Exalted be He) Who enjoins us to only worship him, for He is the One Who has created both human and jinn for the purpose of worshipping as stated in the relevant Ayahs.
Answer: It is prescribed for a Muslim, if he visits the Prophet's Masjid (mosque), to start with offering Salah. If it is possible to be in AlRawdah AlSharifah (The area between the grave and the pulpit of the Prophet), it is better. Then, he goes to the Prophet's tomb and stands before it in a state of submission and politeness greeting the Messenger of Allah (peace be upon him) and his two companions (may Allah be pleased with them) in a low voice.
Whenever someone greets me, Allah returns the soul to my body (in the grave) and I return his greeting.
Allah has angels roaming around; if they find anyone greeting me, they convey these greetings to me.