Is there any first hand evidence, accessible to the public, showing MH17 was shot down?
No, nor should we expect any.
As in almost all cases, the public do not have direct personal access to evidence of the causes of an aircraft loss. For example, It is not possible for me to visit the site of the wreckage and I lack the forensic expertise to make an assessment of causes by examining marks, residues and deformation of the wreckage. I do not have direct personal access to military surveillance systems. I cannot organize my own team of trusted investigators to research the past movements and actions of individual members of various armed forces in the region.
All that we can do is make our own assessment of the likely causes. Lacking expertise and access to the evidence, we have to rely on second or third hand reports. In general, we place our trust in investigation teams and wait for their reports.
Is there any evidence, accessible to investigators, showing MH17 was shot down?
The incident occurred on 17th July 2014. In September 2014 the Dutch investigation team published a preliminary report which says
The damage observed in the forward section of the aircraft appears to indicate that the aircraft was penetrated by a large number of high-energy objects from outside the aircraft. It is likely that this damage resulted in a loss of structural integrity of the aircraft, leading to an in-flight break up.
The pattern of damage observed in the forward fuselage and cockpit section of the aircraft was not consistent with the damage that would be expected from any known failure mode of the aircraft, its engines or systems.
So there exists first-hand evidence showing MH17 was (probably) shot down. However members of the general public do not have direct personal hands-on access to the recovered debris nor to the chain of evidence of custody of that debris that clearly ties it to the crash site, etc.
Alleged evidence reported by news organisations
Prior to these reports, (in this case, at the date the question was asked here,) we rely on reports from news organisations and we weigh what they say according to our personal assessment of their trustworthiness.
If you believe that CNN has reasonable journalistic integrity you might take into account what they report:
A radar system saw a surface-to-air missile system turn on and track an aircraft right before the plane went down, a senior U.S. official told CNN's Barbara Starr. And a second system saw a heat signature at the time the airliner was hit. The United States is analyzing the trajectory of the missile to pinpoint where the attack came from, the official said.
It is of course possible that the quoted U.S. Officials are misinformed or lying to CNN. We each have to use our own judgement to decide how probable we think that is given the circumstances.
ABC news in Australia report
Ukraine's Kyiv Post newspaper has posted what it says is a conversation between a separatist commander and Russian intelligence officer Vasili Geranin.
In the transcript, released by Ukraine's security service, the separatist, identified as Igor Bezler, says: "We have just shot down a plane. It fell down beyond Yenakievo (Donetsk Oblast)."
Of course, it would not be difficult for Ukraininan security services to make this recording using actors. There would be risks for them in concocting a false account but this possibility's likelihood is something we have to assess ourselves.
Other (unsubstantiated) reports - some are now behind a "paywall".
You can never really prove such things to 100% certainty, you can only decide what is the most probable cause based on evidence available at any moment. As new reports of evidence arrive we must reassess the probable cause.