Without an explicit example it is hard to give you some insight in to data cleaning. However, there are common functions that are used to select/convert/transform data:

Part (* to select parts of something based on indices *)
Select (* to select something base on a True/False criterion *)
Cases/FirstCase (* to 'select' something based on its structure *)
ToExpression (* to convert a string to an expression *)
UnitConvert/Quantity/QuantityMagnitude (* to add/remove/convert quantities *)
StringSplit (* split string-data in to parts*)
StringTake/StringDrop (* take parts of strings *)
Map/Apply (* used in conjunction with ToExpression, to convert an entire bunch of items to expression *)
Delete (* delete based on indices *)
DeleteCases (* delete based on a pattern-match *)
DeleteDuplicates(By) (* delete duplicates*)
ArrayReshape/Flatten/Partition/Transpose/Reverse (* flipping/flattening/changing dimensions et cetera*)
Replace/ReplaceAll/StringReplace (* replace items based on replacement rules *)

I think with those, you can get quite far. Of course then you can 'group' the data using:

Gather/GatherBy/GroupBy
Split/SplitBy

Or count items:

Tally/Count/Counts/CountsBy

Sort items:

Sort/SortBy

Then you can do some statistics on it to reduce the data:

Min
Max
Mean/TrimmedMean/Total
Median
StandardDeviation/Variance/RootMeanSquare
Skewness
Kurtosis
Length