Ms Truss opened the session by stressing her determination to bring stability to the economy and provide jobs to the British people.
She was congratulated by several MPs on becoming the country's third female PM.
Ms Truss said: “I am honoured to take my place as Prime Minister in this House and to take on the responsibility at a vital time for our country.
“I am determined to deliver for everybody across our United Kingdom and I will work constructively with all members of this House to tackle the challenges we face.”
When it came to Sir Keir's turn, he opened by asking about Ms Truss' opposition to a so-called windfall tax, which she insisted she maintains.
She added: “I am against a windfall tax. I believe it is the wrong thing to be putting companies off investing in the United Kingdom just when we need to be growing the economy.”
Ms Truss confirmed she will make an energy announcement to the Commons on Thursday.
And, when Sir Keir asked her why she is against taxing big energy companies, she fired back at the Labour leader by saying "this country can't tax its way to growth".
Amid deafening roars from the Tory benches, she added: “There’s nothing new about a Labour leader who is calling for more tax rises. It’s the same old same old tax and spend.”
Liz Truss addressing the House of Commons
House of Commons
Sir Keir then accused Ms Truss of protecting the profits of Shell and giving Amazon a tax break rather than helping families and public services.
He said: “Not only is the Prime Minister refusing to extend the windfall tax, she’s also choosing to hand the water companies polluting our beaches a tax cut. She’s choosing to hand the banks a tax cut.
Liz Truss faced off with Sir Keir Starmer in the Commons
PA
Sir Keir Starmer in the Commons on Wednesday
House of Commons
Liz Truss taking part in her first PMQs
House of Commons
“Add it all together and companies that are already doing well are getting a £17billion tax cut while working people pay for the cost-of-living crisis, stroke victims wait an hour for an ambulance and criminals walk the streets with impunity.
“Families and public services need every penny they can get. How on earth does she think now is the right time to protect Shell’s profits and give Amazon a tax break?”
Ms Truss replied: “I’m on the side of people who work hard and do the right thing.
“That is why we will reverse the national insurance increase and that is why we will keep corporation tax low, because ultimately we want investment right across our country, we want new jobs and new opportunities and that is what I will deliver as Prime Minister.”
Liz Truss addressing the nation for the first time as PM outside No.10 on Tuesday
Aaron Chown
The PM's plans were dubbed a “Truss tax” by SNP Westminster leader Ian Blackford, who said households and businesses will be paying it for years to come.
He said: “The Prime Minister may have changed but it’s the same old being shouted down.
“On her first full day as Prime Minister she has failed to rule out a Truss tax on household and businesses.
"Instead of targeting the profits of massive corporations with a windfall tax, the Prime Minister plan appears to be a decade-long raid on the bank accounts of ordinary taxpayers.
“These costs must not be passed onto consumers and businesses by deferring bills. Government must announce an enhanced windfall profits tax, making sure that those oil and gas producers pay their fair share from excess profits.
“Does the Prime Minister understand that her first act as Prime Minister will now define her? A Truss tax, that household and businesses will be paying for years to come.”
Ms Truss fired back: “Well I’m not quite sure what the right honourable gentleman’s position is, because on one hand he doesn’t seem to want oil and gas extraction from the North Sea, and on the other hand he wants them to pay more taxes. Why doesn’t he make up his mind?”
It comes after Ms Truss' first Cabinet meeting, dubbed "very positive" by new Environment Secretary Ranil Jayawardena.
The new-look Cabinet met on Wednesday morning to consider a plan to freeze energy bills in order to save households and businesses from financial ruin.
Ms Truss will “strain every sinew” to get the economy growing as she looks to "hit the ground running", her deputy Therese Coffey said.
In her first speech as PM, Ms Truss insisted the nation can “ride out the storm” caused by Russian leader Vladimir Putin’s war in Ukraine.
She conducted a major overhaul to leave few survivors from Boris Johnson’s Cabinet, as she failed to find a role for her leadership rival Mr Sunak.
Allies insisted the changes would “unify” the Tory party, pointing to senior roles for five leadership rivals, Penny Mordaunt, Tom Tugendhat, Suella Braverman, Kemi Badenoch and Nadhim Zahawi.
But Dominic Raab, Grant Shapps, George Eustice and Steve Barclay were among the Sunak supporters dispatched to the backbenches.
Along with Kwasi Kwarteng as Chancellor, the appointment of Ms Braverman as Home Secretary and James Cleverly as Foreign Secretary mean that for the first time in history none of the great offices of state are held by white men.
The SNP's Westminster leader Ian Blackford
House of Commons