The same as you do if you can afford dental care:

Most importantly: Brush at least once, better twice a day. No exceptions ever. Make it part of your daily morning or evening rituals. It takes about a day for plaque to harden and then you can no longer brush it away. Don’t ever let it get to that.

If you can afford one, get an electric toothbrush.

Be gentle with the brush. All you need is a very soft scrubbing. Pressure can damage your teeth and gums.

Chewing gum can reduce acids and help remineralize your enamel after eating. Sugar free is better, but afaik even gum with sugar has a net positive effect if you chew it long enough after the sugar has dissolved.

Try to avoid getting into a routine that leaves out spots when brushing. Personally, I switch hands every day and once or twice a week I use a non-electric brush.

If you want to optimize this: those tablets that the dentist uses to stain plaque to check where you missed a spot are cheap, you don’t necessarily need a dentist for that if you can’t afford one.

Floss and use interdental brushes regularly. Use mouthwash every now and then, and at the first sign of gum inflammation. Preferably one without alcohol, because that seems to increase the risk of cancer.

If you can’t afford professional tooth cleaning, try asking for calculus removal, if you have any. Here in Germany that’s ~15€ instead of ~100€. Once it’s gone prevent it from building up again (see above).

If you have cavities, try to get rid of them as early as possible. Maybe there are some charity or teaching clinics. Cavities only ever get more expensive in the long run. And it is much easier to maintain an infection free mouth than to constantly fight to keep the decay from spreading.