My computer is slow during open

Dear All

My MacBook Pro is now very slow to open (start), it may take even 15 minutes to start or open.

What cana i do so that my computer to be fast on open or start as other computers ?

The details of my computer as follows:

Model Name:                           MacBookPro

Processor Name:                   Intel Core i7

Processor Speed:                    2.9 GHz

Memory:                                8 GB 1600MHz DDR3

 Serial Number (system):        C1ML35FFDTY4

Software: OS X Yosemite (Version 10.10.5)

Please help me. it really boring !!

Chanzi, h

Tanzania