Motocross Racing!

My advice to you is to be consistent. Practice, practice, and practice. Also after your race ask to have a copy of your race results so you can provide those to who you want to be sponsored by. Don't get me wrong sponsors do help out, but it will still be expensive no matter what. For a easy way to get some sponsors is making a account on Mxsponsor.com . You pay a yearly membership fee that isn't to much. And you can send request to big name companies on there, and even get offered a sponsorships by companies. Just remember that once you sign a deal to stay true to the company. Do everything they ask plus some. Also ride different tracks, and start doing some local series. If you race a good series that's presented by multiply companies there usually more competitive racers and makes for better racing. And don't ever think that your bike isn't good enough for racing. Everybody knows of Jerry Robin amateur star who raced a 1985 Honda Cr 250r and qualified for Loretta lynn's (One of the biggest amateur races) and placed well at Loretta Lynns. After that Geico Honda gave him to brand new bikes. All goals are achievable if you want it enough. Also you can race your Rm250 in the 250f class, but if you want to race in the 450 class more power to you! Later on down the road if your buying a newer bike from a dealership ask the sales person about a sponsorship, bring in your race results. Most dealers will give you one. it might be only 10% off but nickels and dimes make dollars. Good luck to you...

Yeah man, just keep riding and racing and having fun. Once your results start adding up, a shop sponsor is a great way to start. They know any the local reps from the brands who will be the next step. From there the reps can recommend you to the team managers. Just be friendly, enjoy your time riding and it will come together if you work hard enough. Don't rush anything.

Practice, say that word 1000 times, then go do it 3 times as much.
Honestly, if your 16 rite now and wanna be pro, you better move up to C class, and you Better be winning races ASAP. In other words, now, not later. Only you can make it happen.
MAKE IT HAPPEN
NO GUTS, NO GLORY
WINNERS TAKE ALL

And keep your goal on being a PRO, not a great amatuer. Those require, on some levels, different methods to accomplish. Raw talent and speed make great amatuers, but relentless work and fitness make pros, along with talent. You can get by on the amatuer scene without being in great physical/endurance shape (relatively speaking) is what I mean, but it's one of the biggest factors at the pro level. You still have to be in ridiculous shape to hang at the higher am levels but nothing like what it takes as a pro.

To get any mfg support you have to have a current model bike. As above you need to win and get up to A/B class before you can really expect any real support. All of my exp is in Off road racing and racing for local shops. Most parts mfg's have support go to there web sight and sign up.

Alot of good advice here, and to reiterate, sign up on sites like mXSponsor and Hookit, to try to get some entry level help, as well as your local shop. If you practice, and send them good results, sometimes they will advance your help, as your skill level goes up. (That doesn't mean weekly or monthly by the way, it means yearly,..maybe!) Either way, you have to earn sponsors, and remember to abide by the contract you sign, no matter how measly it may seem! More than anything though, remember that hard work and results are what matter, and how you carry yourself as well. Good luck!