Getting to the Betweenlands is not the simplest task. It is recommended that you have have higher-level gear (preferably, good armor, feather falling boots, and a bow) in order to obtain the item needed to create the portal.
First, you will have to search in the Overworld for a Swampland-type biome. In vanilla, this is always the biome with darker grass and water, trees with vines, and lily pads. Within the Swampland biome you may find a Druid Circle, an ominous location with a circle of runic stone spires and a mysterious altar in the center.
Be careful when you come close to this structure, as you will have to face off against the Dark Druids, a band of magic-wielding hooded figures. The druids attack by levitating and throwing you into the air to cause fall damage, which is why feather falling is recommended; you can also use the water bodies in the Swampland to your advantage in these situations, as not only will the water prevent fall damage, but if you are swimming (not standing) in it the druids cannot attack you. This is where you should use the bow to take out the druids from range.
You should kill enough druids to obtain at least 4 Swamp Talisman Pieces. Afterwards, it is advised to break the Dark Druid Spawner that lies below the altar. From there, right-click the altar and place the four pieces in each of the corner slots. The altar will then play a special animation and combine the fragments to create the Swamp Talisman, the key to creating the Betweenlands portal. After that, simply right-click the altar again and claim the talisman from the middle slot.
From there, it's very simple. Place any type of sapling in a flat and relatively wide area (get rid of the tall grass too), and right-click the sapling with the Swamp Talisman. The Portal Tree will then generate around you, complete with four access portals to the Betweenlands. Make sure you are prepared (do not bring food, potions, torches, or any particularly valuable weapons, for reasons that are explained in the Basic Guide to Survival section). Then dive into the portal to start your adventure in this dark, swampy, and ultimately terrifying dimension.