I wanted to try the elementary os and thought there was only luna and freya, But i knew that the loki will be released in the future, I want to use the latest release always without needing to install another new one after it, I mean if i installed the freya and the loki (not the beta) came out as i know there's no way to upgrade from freya to loki, So if i downloaded the loki (beta) will i be able to upgrade it to the latest release after that ? Or should i just use freya for now ?
I'd recommend using Freya for now. The beta version of Loki is buggy and not ready for everyday use. From the release announcement:
The purpose of releasing a beta is to give developers time to update their apps prior to stable release [...] Beta represents a more-or-less feature-complete release, but it does not represent a stable release.
As far as I know, there wasn't an official upgrade path from Freya beta to Freya stable, and I'd expect the same with Loki. No matter whether you choose Freya or Loki beta, you'll still have to do a clean install to upgrade.
To make upgrading simpler, you could move your home directory to a separate partition. Alternately, you could keep your files in Dropbox or Google Drive.
There are instructions on upgrading from Freya beta to Freya stable, and you might be able to adapt these for Loki. (For example, you'd probably need to change
xenial). That said, I personally wouldn't recommend this approach. It's untested and might result in an unstable system.