As a guy who has worked in the past as a staffer in the House of Representatives and a two year stint with two Senators at the bi-partisan, bicameral U. S. Helsinki Commission, I will tell you that there is no one or easy answer. However, I will offer a few "pointers."
To start, the Constitution requires that you be 30 years old when you are sworn in. So you can be 29 on the day you are elected in November so long as you turn 30 at least 24 hours before being sworn into office in January of the following year. That said, the public tends to like its Senators older and, in any case, it takes a while to get the experience and make the contacts necessary to run for the office.
So that is the easy part. After that, there is no right answer. Typically, United States Senate is not the first office a person runs for. The expenses are not small for one thing.
You have to file with the state to get on the ballot - and most states have some filing fee or such. These vary from small - $100 - to quite large - $2,000 - depending on the state. However, you can do that on your own and it does not take much. Each state has a different procedure, but you can check in with your state's Secretary of State or its equivalent, file the papers and fees and poof! You are on your way.
That is, again, the easy part. The harder part is building a campaign and fundraising. If people don't know you, you are pretty much stopped before you are started. You need to print mailings. You need to print posters and banners. You need to reserve sites to hold rallies. You probably need to hire a pollster to measure how you are doing - NOT CHEAP!! - and also to show polls to potential contributors so that they will invest in your campaign. That all costs money and so you need money to find people with money to contribute to your campaign to buy all that stuff.
Then, usually, debates that get you onstage with your opponents usually each state party has different rules. Mostly you have to have a certain level of support in the polls and a certain number of contributors. So check what the state parties require and start working toward those goals. You need to be in the debates to get on television and radio and get more attention. (Also, you have to be good in campaigning and speaking and all the rest.)
Now, every state will be different. In high population states - think CA and NY - the costs of reaching all the various media markets will be high. In small population states think RI or WY, you can try, to some degree, a more door-to-door campaign. Although in this day and age you still need to reach the media.
So, really, you need to be known to be known. So you can either make a name for yourself in business. Alternatively, you can work for the state party or maybe run for a local office - Mayor of a town or City Council or perhaps for the state legislature or for a statewide office such as Governor. You could also run for a U. S. House seat first or join someone else's campaign in order to meet people. All of these will get you name recognition and if you are good at those positions it will make it more likely that you will get people willing to volunteer for you and contribute to you - and who will find other people to do the same.
So there you have it. Piece of cake. That is the barebones of it. I did not even touch the nuances but that lays out the basics. Hope it helps and best of luck.
Most Helpful Guys