Syed Choudhury, 19, from the Grangetown area of the city, was arrested last December.
The Cardiff and Vale College student spoke only to confirm his name, and to admit engaging in conduct in preparation of terrorist acts.
Originally from Bradford, Choudhury was living with his aunt in Cardiff.
The court heard how in July last year, Choudhury took part in a demonstration in Cardiff protesting about the conflict in Gaza.
He carried a banner proclaiming his support for Islamic State which brought him to the attention of anti-terrorism officers.
Prosecutor Sarah Whitehouse QC said in the weeks following the demonstration, Choudhury accessed websites about travelling to Syria and another one that was entitled "10 reasons to join ISIS".
Ms Whitehouse said he also downloaded images that showed his active interest in extremism and had conversations on social media about travelling to Turkey and Syria to fight.
Choudhury was arrested last December and during interview he repeatedly said he supported IS and that he would be happy to kill non-believers under Islamic State rule.
He was due to go on trial on Wednesday at the Old Bailey, but changed his plea after the jury had been sworn in.
In mitigation, defence barrister Abdul Iqbal said Choudhury did not appreciate the complexities of Middle East politics and he had been deeply influenced by older men in the Cardiff area who he considered to be more learned than him.
Mr Iqbal said the defendant was immature, "not the most academically capable" and had showed "immaturity and a lack of sophistication".
Judge Peter Rook QC adjourned proceedings until 7 July for a pre-sentence report to be compiled.
"It is quite clear you wanted to travel to Syria and join IS and the only reason you had not is because you had not found someone you trusted to travel with," he said.
The judge said a custodial sentence would be passed and the issue of "dangerousness" would be considered.
A Cardiff man who planned to travel to Syria to fight with Islamic State has pleaded guilty to a terror charge at the Old Bailey.