This seems to be an area where many of my students get very confused.
The first thing to understand is what the MSA is. It is an altitude, at or above which you are guaranteed to be safe from hitting the terrain. That does not mean it's not safe, legal or correct to fly below that altitude. It does not mean it is legal to fly, or plan to fly, at or above that altitude (at least not without the correct permission).
Take, for example, the Manchester Low Level Route. The route must be flown not above 1250'. Many of my students who do not understand the concept of MSA correctly therefore think that the MSA must be below 1250'. This is not true. In the case of the LLR, the MSA is something above 1250' (I don't have a chart to hand to confirm exactly what it is).
You may plan to fly the route at 1100', which is both safe and legal, but below your MSA. If, as you are flying down the route, you inadvertently enter clouds, your first reaction should be to note that you are below the MSA, and should therefore climb to the MSA in order to be certain of not hitting anything on the ground. Of course, this climb will take you into controlled airspace, so your next reaction should be to inform Manchester Approach that you have inadvertently entered IMC and are climbing. (A Mayday would be appropriate in this situation if you are not qualified to fly on instruments.)
Ok, so now that's cleared up, how do you calculate the MSA?
As Scuba said, there are 2 basic methods. The first involves using the Grid Maximum Elevation figures. These are the big blue numbers on your chart - one in every latitude/longitude rectange. This will give you the highest known elevation anywhere in the rectangle, so find the highest one, and then add 1000' on to that to give you a safe altitude.
The second method involves tracing along your route, finding the tallest object within a certain distance of your track. This distance should be at least 5nm. This method takes a little longer, but gives a more realistic and less conservative figure. Again, you need to add 1000' to the highest object you find to give you a safe altitude.
But there is an extra complexity with the second method, and this is the 300' which Scuba mentions. The chart designer shows you any structures which are more than 300' tall, so if there is nothing shown on your chart you can assume that there are no structures taller than that. But there may be a 299' structure on the highest piece of ground you identify. So you should add 300' to all ground elevation figures to take account of this. On the other hand, if your track takes you past tall masts which are marked on the chart, there is no need to add this 300', because there will not be a 299' structure built on top of a mast.
If you use the first method of calculating the MSA - using the Maximum Elevation figures from the chart - there is no need to worry about adding 300', because the chart designer has already done this for you.
FFF
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[Edit to note that in the time it took me to type this, a couple of other replies appeared - I agree with all of them]