There are many reasons why a tyre might be going soft, ranging from cheap and easy to fix right up to buying new wheels.
Find a good independent tyre fitter in your area - ask friends/family if they have someone they can recommend. Fast-fit places will probably want to sell you new tyres without doing any real fault finding.
Possible causes of pressure loss:
Loose valve stem insert - it can work loose, or might not have been properly tightened last time around
A valve spanner is only a few quid in Halfords and it's something you can check yourself. It's usually my go-to first fix for a tyre that's going soft
Related to the above is perished rubber on the valve stem
Solution is to replace the stem - job for a tyre fitter
Poor seal between the tyre and the rim: the tyre may not have seated quite right when fitted, or there was dirt left on the rim, etc
Fix is to reseat the tyre after checking the bead and rim carefully - job for a tyre fitter
Both of the last two could/should be done together before going further: it won't cost a huge amount of money and fixes most problems with tyres losing air
Slow puncture in the tyre: it can happen from a piece of glass, or a nail, etc. If the object stays stuck in the tyre it'll plug the hole enough that it'll only leak slowly
A tyre fitter should be able to find if this is the problem, and unless the damage is bad, the tyre can usually be safely patched inside
This has saved me from buying new tyres more than a few times
Damaged wheel:
As Carlston pointed out, a clumsy tyre fitter could damage the tyre seating surface on the rim making it difficult to achieve a seal
Badly kerbing the wheel could do this too, or warp the wheel to the point where the tyre can't seat properly
Alloy wheels can crack; if this happens on the pan or rim it will leak
The fix here is refurbish the wheel - it should be about 1/4 to 1/3 the price of a new wheel from a competent repairer (I had mine refurbed last year at €70 per wheel including fitting of new tyres)
Unless the wheels are in really bad shape, or you just want rid of them anyway, buying new wheels is a waste of money right now.
If you do want to buy new rims, the following are the type approved sizes for the 1.8 TSI Facelift for year round use:
205/55/R16 91V on 7J x 16 ET45 rim
205/55/R16 94V on 7J x 16 ET45 rim
225/45/R17 91V on 7J x 17 or 7.5J x 17 ET49 rim
225/40/R18 92V on 7.5J x 18 ET46 rim
For winter use only these additional sizes are approved:
205/55/R16 91V on 6J x 16 ET50 rim
205/50/R17 93V on 6J x 17 ET45 rim (this isn't formally restricted to winter use in the tables, but given the unusually narrow wheel/tyre it seems likely that this is the intended use)
The winter sizes allow more room for use of snow chains in regions where that is required.
Any sizes not on these lists are not type approved by Skoda. If you want to go outside these, it will require additional due diligence on your part to ensure any wheel/tyre combination won't affect ride/handling/safety/speedometer. Changes to profile height and/or rim width and/or offset (ET) will affect scrub radius, which affects steering and handling. Your insurer will probably need to be told too if you go with a non-type approved size.
One exception is you can choose a size listed for a more powerful version of the car e.g. 2.0TFSI. Mostly the only difference is the speed rating, and possibly load rating.