You may or may not get a fruit, it depends on whether or not the flower was pollinated. If you do get a fruit, wait until it is nice and ripe and detaches easily from the plant. You should then leave it to dry out thoroughly and then somehow separate the seeds from the pulp. Different people have different ways of doing that, so wait for them to chip in. Personally, I tend to try and remove any pulp by hand, and that is probably the best option for you as well, since you're only dealing with one fruit. After you've obtained the seeds, you just sow them as you would normally. There's more information of sowing cactus seeds on this forum. Try reading some old posts if you want.
Hope that helps!