|What make and model computer do you have? That information will allow folks here to provide help that is specific to "your" computer.|
Wifi can be simply "turned of" on your computer - but with the above information it will be easier to advise you exactly "how".
If you are using wifi and your router wifi access is not password protected - the very least you need to ensure… - then others can both use your service (and likely slow you down on the www, as they will be using some of your bandwidth); and if they know how.. can possibly hack into your computer too.
If you are using only the wired (ethernet/cat-5) connection then you are essentially safe from those problems. Your computer will ideally still have a firewall enabled (typically M$ Firewall - by default - and which is part of most windows OS these days?); and your router will likely have a built-in one enabled too. Between them they will help keep most uninvited guests out…
BUT if you inadvertently download/install trojan viruses and other pests… then they can circumvent (get around) your firewall settings and get upto all manner of "mischief"… Which is why you also must have some form of anti-virus/malware protection too.
If you are the only user of your computer, then it's not really necessary to have a logon password… That is if you're absolutely certain that no-one else will ever access it and decide to use it for whatever purposes… However if you have any doubts about that, then yes set a password to go along with your user-name; and then you will have to provide that password too - to logon etc.
But.. be sure you can remember what the user-name and password are, and also perhaps have a safe record of them in more than one place (hard-copy/printed out too). I often email it to myself - in a form I can understand; as then its on my on-line email account server (e.g. outlook.com, yahoo.co.uk, gmail etc… whichever you may decide to use) - should I need it?
The wifi button is the usual way to disable it… but with the details of you make/model etc. we likely can suggest how to make it a little more definite - until you want it back again…?