If you're just getting started and want to build a double oval, flex track might be overkill. What you want is sectional track (e.g. Peco Setrack), which offers track pieces in matching geometry and set lengths and curve diameters. For a double oval you might commonly use R3 curves (585mm radius in the Peco geometry) for the outer oval, and R2 curves (438mm radius) for the inner track. If you already purchased track, i.e. with a starter set, you want to make sure that any additional track you're buying is the same make and height (or Code).
Hornby starter sets usually come with an oval, and they offer several extension packs to easily extend that oval. Hornby track largely uses the same geometry as Peco Setrack, so you can mix and match both brands to a degree.
To connect both tracks, you need turnouts (also called points or switches).