Let's say I wrote a good screenplay and I want to go into the cinema work. What should I do? and to who should I go to?
Also, If I made a short film and a lot of people liked it, how can I let the companies or whatever watch it? Is there any topic or something for those who are interested in the movie work and wanna go into it?
You need a crew, and you may be able to find one via Craigslist. If you're lucky this crew will come with their own camera, sound gear, editing software, etc. A point of advice, though - and I say this from experience - you generally cannot rely on a crew that is not being paid. Educate yourself on how to properly crowdsource and put up a campaign on Kickstarter. But be VERY realistic about your budget, your needs, and your goals.
If your kickstarter is successful it will give you financing and buzz. You'll also need to market the crap out of your film if you want anyone to see it. Create a website, enter as many festivals as possible, and keep at it. It could literally take years but if the film is any good the right people will see it.
A side note too - network as much as possible. I've been fortunate enough to work with some pretty substantial movie stars and I've seen my work go on nationally-distributed DVD's and this is solely due to connecting with the right people at the right time.