Make the lines of a description item line up

If i did understand you correctly, the labeling environment provided by the KOMA-script package does the job you want:

\documentclass[12pt,a4paper]{article}
\usepackage{scrextend}
\usepackage{blindtext}
\setkomafont{labelinglabel}{\bfseries}
\usepackage{showframe}
\begin{document}
\begin{labeling}{A long long label}%Giving the longest label for alignment
    \item [Label] 
        Here is a long item that requires a line break. The second
        line of this item won't line up with the first word. I can
        achieve this manually\\
        by forcing a line break and setting an apropriate value in
        \textbackslash hspace*, but I want this to be automised.
        Please help me.
    \item [A long long label] 
        a manual linebreak, though not
        recommended \\
        \blindtext
\end{labeling}
\end{document}

Here package scrextend provides all you need, switching to a koma class (scrartcl) could be an advantage. The labeling environment takes the longest label as the argument, needed for the right alignment. This argument can also be shorter, just play a bit.

As an alternative to a KOMA class or to scrextend, you can use the enumitem package, setting the key labelwidth to the width of your label.

I got this solution by combining (a) Bernard's answer to Simple chronology using the description environment and (b) Bernard's answer to How to set leftmargin of description to width of a particular label in enumitem?. You can find the same solution in Gonzalo Medina's answer to Description list with aligned descriptions.

\documentclass[12pt,a4paper]{article}
\usepackage{enumitem}
\begin{document}
\begin{description}[%
        labelwidth=3cm,%
        leftmargin=!%
    ]
\item [Label]Here is a long item that requires a line break. The second line of this item 
won't line up with the first word. I no longer need to achieve this manually by forcing a
line break and setting an appropriate value in \textbackslash hspace*.

Here's a second paragraph of the same item.
\item [A longer label]Here is a second item. It has a longer label, necessitating a larger labelwidth.
\end{description}
\end{document}

Here's the output:

You can also calculate the width of the widest label by adding the calc package and using its \widthof command:

\documentclass[12pt,a4paper]{article}
\usepackage{enumitem}
\usepackage{calc}% Supports \widthof command
\begin{document}
\begin{description}[%
        labelwidth=\widthof{\bfseries A longer label},%
        leftmargin=!%
    ]
\item [Label]Here is a long item that requires a line break. The second line of this item 
won't line up with the first word. I no longer need to achieve this manually by forcing a
line break and setting an appropriate value in \textbackslash hspace*.

Here's a second paragraph of the same item.
\item [A longer label]Here is a second item. It has a longer label, necessitating a larger labelwidth.
\end{description}
\end{document}

This gives essentially the same output. Note in \widthof{\bfseries A longer label}, the necessity of specifying \bfseries: The label is typeset as bold. You need to make sure that the text whose width you measure with \widthof is also typeset as bold.

You can go a step further in automation by having the environment automatically determine which label is widest. See Gonzalo Medina's answer to Automatically set description list labelwidth based on widest label?.