First, I advise you NOT to go on social media that don't allow adult content and start linking to adult sites. Many of the guys here are using fake profiles and ip addresses, so if they get in trouble it's no big deal. If you get in trouble, it could be a big deal. Unless the platform you use has an option to label your profile as mature, don't bother using it.
With only $300 dollars you should try to maximize free methods first. Make accounts on xvideos, pornhub, and reddit for starters. Now you have to decide on some niches you want to provide content for. On reddit, the best niche for getting fans is the r/nsfwcosplay It has a lot of competition and the guys can be picky, but if you are creative, you can build a following. Try to find some other niches to cross with your cosplay and then you are set. There are certainly other niches besides cosplay that work though. If you don't want to do cosplay, r/camsluts is another friendly sub that has users who are active on cam sites. Gifs seem to do best on reddit instead of video and pics. Just try to find some kind of fetish to provide for.
After deciding what niche you will do, start making free content for reddit, pornhub, and xvideos (picture sets, teaser videos, and finally your subscription sample). The subscription sample should be like 1 out of every 25 videos. Something like "Misty does xxx - Onlyfans Sample". Have an intro and outro that tells people about your Onlyfans account and exclusive content. Get a custom domain that redirects to your Onlyfans account.
Some other things to note is that reddit takes a bit of learning. Each sub has its on rules. The r/nsfwcosplay and r/camsluts subs are pretty chill when it comes to self-promotion but other subs don't even allow watermarks. People will click your username if they are really interested though. Make sure to create your own subreddit when you get enough karma. Sticky your profile links to the top and fill the subreddit with free content to gain followers. Link your subreddit in comments only on your own posts. Also, aim for quality more than quantity. Reddit users can be picky and really take the subcategories seriously.
Lastly, these suggestions also work for stuff like Manyvids and Modelhub. You might want to do those instead of Onlyfans. Most guys here would love to be a beautiful woman because you are essentially your own supply of promotional material. It all depends on how you want to promote yourself.
This is just what I would do. I'm not some big fish, so take it for what you will. I have grown an adult subreddit to nearly 30K subscribers though so I'm not talking completely out my ass.