Entering the portal is easy, but it has downsides. You simply need to notify your current coach that you wish to transfer. He will coordinate with your school's compliance department or refer you to that person. You will fill out some light paperwork and then your name will hit the portal within a couple of business days.
Now that you are in the portal, your days with your current team are over. The coach will remove you from the active roster, you will not be permitted to work out with the team nor use any athletic department facilities. The school will terminate your athletic aid agreement immediately. Do not take it personally. It's just business. That said, you might want to hold off until after the track season so that you can complete the academic year on your current aid agreement. That also goes a long way towards protecting your reputation and quitting midway through your freshman year can raise some assumptive alarms with prospective coaches (Ex. - couldn't hack college classes, homesick mama's boy, girlfriend troubles, creates team drama, broke team rules, etc.). None of that may be true, but some people will jump to conclusions or assume that you are not dependable or loyal.
If you enter during the competitive year, then you will be racing unattached and on your own dime. You will not receive any logistical support and, depending on where you are, gaining entry to meets will be harder. Many big invitationals will only allow entries from college teams and your high school times (the only verified marks to your name) might not be fast enough to get into open heats. Road marks, like XC times, are generally not considered for recruiting due to lack of course certification and accuracy.
Once in, opt to allow coaches from other programs to contact you. This signals that you are open to recruiting and you will start to get a lot of attention. My son was in a similar situation to yours and heard from roughly 30 programs within 48 hours of entering the portal. There was more interest after a year of college running than he received coming out of high school. Be transparent when talking to prospective coaches and NEVER say bad things about your current school or staff. You never know if the coach that you are chatting with is good friends with or worked alongside your current coach. It's a very small world.
Best of luck with your decision!