Once you have reached Player Level 10 and proposed you will need to pick up several items to get married. At the 'Outdoors' category of the 'Shopping' menu you need to buy a Wedding Arch, Floral Pillar and Windowed Rose Lattice. Then before the wedding you need to complete the following requirements.
3 Friends:
Your Sim needs to have a minimum of three people on Level 2 relationship under 'Friend'.
Reassure Partner:
Simply speak to your Sim partner and choose the option to reassure them.
Wedding Vows:
Interact with a table and choose the option to work on your wedding vows.
All you need to do now is interact with the Wedding Arch, select the 'Get Married To' option, and select your life partner.