I have problems translating this table (from Online table generator) to tabularx environment so it would be coherent with the rest of Tables of my document. Could you kindly help me? Thanks!

documentclass{article}

usepackage{multirow}

usepackage{graphicx}



begin{document}

begin{table}

centering

caption{My caption}

resizebox{textwidth}{!}{%

begin{tabular}{cccccc}

hline

1                     & 2                    & 3                    & 4                    & 5                    & 6                    \

hline

multirow{2}{*}{Pepe} & multirow{2}{*}{a}   & multirow{2}{*}{b}   & multirow{2}{*}{c}   & multirow{2}{*}{d}   & e                    \

                      &                      &                      &                      &                      & f                    \

hline

multicolumn{1}{l}{}  & multicolumn{1}{l}{} & multicolumn{1}{l}{} & multicolumn{1}{l}{} & multicolumn{1}{l}{} & multicolumn{1}{l}{} \

hline

end{tabular}%

}

end{table}

end{document}

That is, the table should start by:

documentclass{article}

usepackage{multirow}

usepackage{graphicx}

usepackage{tabularx}



begin{document}

begin{table}

begin{tabularx}{textwidth}

end{tabularx}

BLA, BLA, BLA.

end{table}

end{document}

Here are two solutions. The first uses a tabularx environment, with all six columns using a centered version of the X column type. The second uses a tabular* environment, with all six columns using the c column type. Both solutions occupy the full width of the textblock.

Since multicolumn{1}{l}{} does nothing at all, you might as well omit those instructions.

documentclass{article}

usepackage{multirow,tabularx,booktabs}

newcommand{mr}[1]{multirow{2}{*}{#1}} % handy shortcut macro

% centered version of X column type:

newcolumntype{C}{>{centeringarraybackslash}X} 



usepackage[skip=0.333baselineskip]{caption} % optional



begin{document}

begin{table}

%%centering  % <-- redundant

caption{tabularx}

begin{tabularx}{textwidth}{@{} *{6}{C} @{}}

toprule

1         & 2      & 3      & 4      & 5      & 6 \

midrule

mr{Pepe} & mr{a} & mr{b} & mr{c} & mr{d} & e \

          &        &        &        &        & f \

midrule

  &  &  &  &  &  \ % no need for "multicolumn{1}{l}{}" stuff

bottomrule

end{tabularx}



vspace{1cm} %

setlengthtabcolsep{0pt}

caption{tabular*}

begin{tabular*}{textwidth}{@{extracolsep{fill}} *{6}{c} }

toprule

1         & 2      & 3      & 4      & 5      & 6 \

midrule

mr{Pepe} & mr{a} & mr{b} & mr{c} & mr{d} & e \

          &        &        &        &        & f \

midrule

  &  &  &  &  &  \ % no need for "multicolumn{1}{l}{}" stuff

bottomrule

end{tabular*}

end{table}

end{document} 

As @daleif mentioned you forgot to add the second bracket with your columns' specification {cccccc}.
I give you a MWE:

documentclass{article}

usepackage{tabularx}



begin{document}

begin{table}

caption{BLA, BLA, BLA.}

centering

begin{tabularx}{.3textwidth}{c}

hlinehline

My table header\

hline

My table contents\

hlinehline

end{tabularx}

end{table}

end{document}

Supplement

Additionally, keep in mind that there is a package called tabu, which is more powerful than tabularx.

You do not need multicolumns in every cell, nor multirow{2}{*}{...}, nor tabularx or tabular* to extend the text width, nor empty rows. Here you have enough with m columns (of array package) and, of course, with booktabs: :)

documentclass{article}

usepackage{graphicx,array,booktabs}

begin{document}

begin{table}

caption{My caption}

newcolumntype{C}{>{centeringarraybackslash}m{dimexprlinewidth/6-2tabcolsep}}

begin{tabular}{CCCCCC}toprule

1  & 2 & 3 & 4 & 5 & 6 \midrule

Pepe & a & b & c  & d &epar f\bottomrule

end{tabular}

end{table}

end{document}

By the way, I would not use columns of more two-three em here. Widening the table as much as possible instead of a small centered table does not make it more beautiful, but ugly scattered.