Don't know particularly for a fiesta but ...
As its over three years old it should be MOT'd every year (or VOSA) usually serviced and MOT'd together, otherwise its illegal to drive and the ANPR cameras will flag you up every time
Usually replace the air filter, can get from halfords, or ebay or local motor factor, whatever you do don't remove it and run the engine without one, it will be great for a while then B*** expensive to repair.
Some you can clean by washing with petrol, but they are usually paper and that would destroy it, if its a cloth sort of thing then washing with petrol is the usuall way, but let it dry out before you try running it.
Usually follow the air intake pipes and you'll come to a plastic box that holds the air filter, sometimes held closed by spring clips or screws. Air intake is other side of engine from exhaust.
If its misfiring, could be that or spark plugs, you can remove and clean them, or replace them. You will need a set of feeler guages to re-set the gaps and a plug wrench to remove/ replace them.
For the oil change - no point unless you also replace the oil filter, just keep it topped up, but on no account overfill or you end up bowing the end gaskets.
To change the oil you need it up on ramps, or over a pit, or on a lift, or jack it onto axel stands. There should be a bolt (or maybe a plug with a hex socket) on the very bottom of the engine, removing this will let the oil out. This should be done with the engine hot to get best drainage. The oil filter should be somewhere on the side of the engine, looks vaguely like a coke can but with a rounded bottom, sometimes a hex nut on the bottom, sometimes you need a filter wrench to remove them. Some have a heavy casing and you replace an insert, some are lighter (cartridge filter) and you just unscrew and screw in a new one. Always use the new seals that come with the filter., sometimes you need to scrape the old seals off, but usually not to hard.
The local tech colleges used to do a motor maintenance evening classes that cost next to nothing and will give you enough nonce to not get screwed by the garage recommending unnecessary or inappropriate repairs.
Another good tip is to go to the local motor factors and blag your way as trade, get name and address of local back street garage and say they asked you to pick up the parts. I got 45 quid oil (0w30) for 13 quid etc...
Woops, bit long of an answer
Father was mechanic, used to rebuil engines and gearboxes, now IT engineer, run my own car 40,000+ miles per year - just buy a cheap one, fix it, drive it and sell it.