Citrus trees from seed don't come true and may take many many years to produce flowers and fruit, and sometimes never! And if it does fruit, the result is likely to be inferior.
Do not despair or throw away your plant though!
What I would do is see if you have a friend or neighbour who has a fruiting cultivar in a pot which you can borrow for a month or two. (Offer them some wine or a few plants in return!)
Place your plant and the fruiting cultivar close together so you can bend two young branches/stems (one from each) into close proximity.
Carefully remove the cambium (the green layer on the stem) from one side of each of the stems and then bind them closely together with tape. They must make close and intimate contact. I've used sellotape and it worked.
Leave the ends of each stem/branch with their growing tips, as you don't cut them off until later.
After a month or two, you should find that the two stems have fused together. Very carefully cut the fruiting cultivar's branch away from its parent, and return the plant to your friend.
The fruiting stem which you have grafted will form the basis of your new fruiting lemon plant, and you can cut the other stems off once you are sure your graft has taken.
This is called proximity grafting and is the easiest type of grafting to do, as both plants remain growing while the graft takes. It must be fool proof as it's never failed for me, and we have created about five fruiting lemon plants from seed grown citrus plants this way.
I've explained this very badly I think, but you get the basic idea and you can always Google 'proximity grafting' to get a better more detailed explanation.