First you will need your 8 digit zip code
Once you have that, are you contacting your US Congressmen
Or your State Legislature and/or Governor?
I wouldn't recommend contacting the president, that's a good way to get put on a watch list. I also highly doubt he will even see what you have written, where as the legislatures and governors most likely will.
Once you've got that sorted out, follow the instructions on each website. It's pretty simple. You need your 8 digit zip code for federal and state representatives because their districts change frequently, and you can be represented by one member, and your neighbor on the other side of the street may be represented by another. It's important you contact the member who actually represents you.
It's irrelevant if you voted at all, voted for them, or voted against them. They don't know any of that, you don't need to disclose it, nor should you. All that matters is you live in their district and your telling them what to do on a specific issue.
Remember that you don't need to be eloquent or verbose, nor do you need to lay out an argument to justify your opinion. They are your servant and you must treat them as such. Be respectful, but tell them in no uncertain terms what you want them to do. If the issue is a big enough concern, make sure you tell them that their failure to comply will result in you voting for their opponent next election. If it's a really big deal, and you really mean it, that you will also actively help their opponents campaign. Don't do that for things that aren't worth it though.
You vote with the voice of one, but you speak for hundreds, if not thousands when you email your public servants. If as little as 50 people email them about an issue, they know it's a big deal. Hardly any one contacts them, so your voice WILL be heard, and it will matter. Spread the word.