Can I take position behind a plane for V formation legally?
Here are the regulations in 14 CFR 91.111:

§91.111   Operating near other aircraft.
(a) No person may operate an aircraft so close to another aircraft as
to create a collision hazard.
(b) No person may operate an aircraft in formation flight except by
arrangement with the pilot in command of each aircraft in the
formation.
(c) No person may operate an aircraft, carrying passengers for hire,
in formation flight.

&quot;Formation flight&quot; is defined in the Pilot/Controller Glossary:

FORMATION FLIGHT− More than one aircraft which, by prior arrangement
between the pilots, operate as a single aircraft with regard to
navigation and position reporting. Separation between aircraft within
the formation is the responsibility of the flight leader and the
pilots of the other aircraft in the flight. This includes transition
periods when aircraft within the formation are maneuvering to attain
separation from each other to effect individual control and during
join-up and breakaway.
a. A standard formation is one in which a proximity of no more than 1
mile laterally or longitudinally and within 100 feet vertically from
the flight leader is maintained by each wingman.

And there are ATC instructions for handling formation flights but they're too long to quote here; see the ATC Orders 2-1-13.
So, let's assume that the distances between the aircraft in your proposed flight would make it a formation flight, per the FAA's definition in the PC/G. Putting all that together:
As a pilot, what would I need to locate other planes near me? Or find and [wait] on a plane going in the direction with equal cruising speeds to follow?
No idea. This depends very much on the type of aircraft and operation. Two friends in C172s flying half an hour for lunch somewhere? Two Boeing 777s flying internationally? But practically speaking, this would be near impossible without prior coordination on the ground. How would you know what route the other aircraft is following, or what cruising speed they plan to use?
Then would there be [an] issue of just taking position without asking. Should I ask for permission to do so, from the pilot which I'm shadowing or no I need more?
Per 91.111(b) you must have an arrangement with the other pilot first. In theory you could do that in the air but people who do formation flying seriously - and safely! - spend more time on planning than on flying. I don't know many pilots who would form up in formation with a stranger who just asked.
If you're thinking of using the V formation for commercial passenger flights then see this question. Although as you can see from 91.111(c), it isn't legal today, even if the pilots coordinate and work out all the necessary procedures first. And that's without even considering what their employers and insurers would say about it.