Dear Martin,
The solution is simply
TTest[data,zaklad,SignificanceLevel->0.05]
to get the p-value or
TTest[data, zaklad, "TestConclusion", SignificanceLevel -> 0.05]
to get the conclusion:
The null hypothesis that the mean of the population is equal to 8 is not rejected at the 5. percent level based on the T test.
For more details please refer to: https://reference.wolfram.com/language/ref/TTest.html
If you want to perform the test yourself, it is required that you count the test statistic and compare it to the quantile of the t-distribution corresponding to the
$\alpha =0.5$ critical values.
data = {7.8, 7.9, 9, 7.8, 8, 7.8, 8.5, 8.2, 8.2, 9.3};
zaklad = 8;
hladina = 0.05;
n = Length[data];
mi = Mean[data];
sigma = StandardDeviation[data];
t = (mi - zaklad) Sqrt[n]/sigma;
qt = Quantile[StudentTDistribution[n - 1], 1 - hladina/2];
If[Abs[t] <
qt, "No basis to reject the H0: mi=zaklad", "Reject the hypothesis H0: mi=zaklad"]