Each year, Y Combinator accepts a batch of startup companies to work with. Startups work closely with a partner at Y Combinator for 3 months, during which they receive intense support to help them get on their feet. This includes access to office hours with experienced startup founders, investors, and mentors, as well as free hosting, legal services, and more.
At the end of the 3 months, startups give a final presentation to a room full of investors, who then decide whether or not to invest. Y Combinator has a successful track record, with hundreds of startups that have gone through the program and gone on to be extremely successful.
If you're interested in applying to Y Combinator, you can do so here.