Start doing:
Pay attention to your health - a good amount of exercise, and a balanced diet (yes that includes some treats). As you get older it's so much harder to get fit if you're not already there. Getting fit early and maintaining it is much better.
Similarly, get to know your body. Know what you look like, what is normal for you, so that you notice changes.
Educate yourself about menopause. Perimenopause creeps in earlier than most people realise.
Do a life MOT and start making plans/setting goals. Consider what you are earning, what your earning potential could be in future and what you need to do to get there. Get into a pension plan. Where your career is, do you want to progress up, do you want a career change. Think about what you want to spend on in future - do you want to buy a property, or upsize/downsize, do you want to travel, do you want a career break for study or fun, will you take maternity leave. Do you want to marry/date/not, do you want children.
You don't need to do all of these things now, and you may change your mind later, but it's good to focus and give some time to thinking of your future. At 35 I was 15 years into my career, I knew I wanted to change career, so I started a plan of training and development that meant 5 years later I was in a different place - planning ahead can pay off.
Stop doing:
Stop investing time in people who don't bring you any happiness. Those people you are friends with simply because you've known each other a long time but you just don't really like them.
Stop procrastinating, stop thinking of doing things or things happening "one day". At 35 you are statistically half way through your adult life - one day is now.
If applicable, stop criticising yourself. Those negative thoughts we hold in mind - I'm too fat to wear this, I'm not interesting enough to be friends with that person, I'm not experienced enough to apply for that job. Be your own cheerleader instead.