To marry anyone, you have to speak with Maramel in Riften and do this Skyrim:The Bonds of Matrimony - UESPWiki wear the Amulet of Mara, and go talk to Lydia. To marry Lydia, you have to complete the quest "Dragon Rising" and be the owner of Breezehome in Whiterun.

Is Lydia good to marry?

Considered among the most popular companions and marriage partners, Lydia is an iconic spouse in Skyrim. She's relatively easy to meet since the main quest will lead players her way.

Can you marry Lydia if she is a steward?

Yes you can marry your steward. I have Lydia as my steward at Lakeview Manor but there is no dialog for marrying her at all, just steward dialog and housecarl dialog.