Textbox validation: alphabet and numeric but always start with alphabet not numeric

Use a Regex:

^[a-zA-Z]\w*$

should do it.

if (!val.match(/^[a-zA-Z]\w*$/)) {
$("#lblErrorMsgDatabaseDispatchers").text('');
$("#lblErrorMsgDatabaseDispatchers").text('Please ClientId Start with alphabet!!!');
$("#DIV_ValidationAlertError").css({ display: "block" });
return false;
}
else {
$("#lblErrorMsgDatabaseDispatchers").text('');
$("#DIV_ValidationAlertError").css({ display: "none" });
}

//// its work

We do not do your HomeWork.
HomeWork is not set to test your skills at begging other people to do your work, it is set to help your teacher to check your understanding of the courses you have taken and also the problems you have at applying them.
Any failure of you will help your teacher spot your weaknesses and set remedial actions.
So, give it a try, reread your lessons and start working. If you are stuck on a specific problem, show your code and explain this exact problem, we might help.

Learn how to use the regular Expressions !
Here is a link to RegEx documentation:
perlre - perldoc.perl.org[^]
Here is links to tools to help build RegEx and debug them:
.NET Regex Tester - Regex Storm[^]
Expresso Regular Expression Tool[^]
This one show you the RegEx as a nice graph which is really helpful to understand what is doing a RegEx:
Debuggex: Online visual regex tester. JavaScript, Python, and PCRE.[^]