hi ollie - and welcome to T2W
You don't really calculate (well, I don't anyway) support and resitance levels - you look at a chart and see them.
They're the peaks and troughs of previous price action (price bars), and the bigger the peak (or trough) usually the more important it will be.
It also depends on your timeframe - on intraday charts the more important support and resistance levels are horizontal over the last 2-3 days usually, whereas on end of day charts the support and resistance levels can be going back years. There may also be angled support, where a trendline can link two or more points.
You'll also find that significant numbers (such as 800p, 200p, 1000p etc) are also support and resitance points as these are psychologically important because shareholders remember these points. Price has memory, and you'll find that is why support and resistance points work so well - imagine that you narrowly missed selling a stock at its all time high of, say, 225p. So next time it rises to 224p or 225p you will probably sell, and as hundreds of shareholders might be thinking the same, that will make 225p a resistance point.
Is there any particular stock you're interested in for support and resistance points?