There are lots of ways. Here are seven:
1) You can write him a letter. You know, on paper, in an envelope, with a stamp.
2) You can send an electronic message
3) You can call
To do any of these see here
4) You can start a petition
5) You can tweet
6) You can blog
7) You can stay home on election day.
What are the advantages and disadvantages of each?
Well, writing (1 or 2) let you carefully craft a message, reference specific legislation, edit your words, and have a record of what you said. But they take time and are solitary (it's only you ..... although others may be doing the same, you don't know)
Phoning (3) is more immediate. You get to speak to someone. OK, it won't be Obama or a senior person, but it will be a person, and it's quicker than a letter. It is also solitary. Unlike a letter, you don't get to edit so much.
Petition (4) could gather a lot of names, and can be specific, but it's hard to get a lot of detail, or people won't read it, much less sign it. And it's labor intensive.
Tweeting (5) is very fast and easy, and your tweets may get retweeted; some of my tweets get retweeted dozens of times. You're limited to 140 characters, though - but you can add links.
Blogging (6) lets you get as detailed as you want, but it's hard to get an audience unless you're famous, or unless you are very good and lucky. You can combine blogging and tweeting though.
Staying home on election day (7) well, what message does this send? Who knows? Do you think the president (or anyone) can read our minds? Oh, Joe stayed home because Obama signed NDAA, and Bob stayed home cause he was sick, and Jane stayed home cause she was only going to vote for Paul. Carol wanted us to be more liberal. Ted wanted us to be more conservative.
Who knows? No one.