Can't shift to gear one or two I had an issue where every time I put my bike in gear 3, it would just keep "skipping". By this I mean that it wouldn't stay in gear 3; the chain would go onto it, then fall onto gear 4, then go back onto it, then back on 4.

Some Googling showed me that I might need to adjust the rear derailleur. So I set about doing this today by first putting my bike onto the biggest cog and adjusting it so the derailleur was in line with the cog. Then I shifted it down to the smallest one and did the same thing (I think I got a bit confused between this and changing the screws to line these two up with something).

Anyway, as luck would have it, this fixed the problem of the skipping however I now can't shift into gears one or two (the two biggest cogs). Like I literally can't flick down the gear lever to put it into these gears.

What could be the issue? I would love some advice.

Thanks
If you were able to shift into all of the gears prior to your adjustments then it would make sense that you need to readjust.  It sound like you you have adjusted it too far outboard.  I would take it in significantly so that when in 1st gear position on the shifter you are on the largest cog.  From there you can make fine adjustments for shifting between gears.