Beginners's Guide

This is answererd easily: You will need a Yo-Yo and Replacement strings.
To get a precise recommendation on what to buy, you need to answer 2 questions though:
1.) How old is the person you are buying the yo-yo for?
2.) What is your budget for yo-yo and strings?

I recommend yo-yos with "tug response" (this means that the yo-yo comes back with a simple flick of your hand) for children. This way, they can learn the basic skills needed and get a better feeling for the yo-yo and the first few tricks. This will make for a much more enjoyable start.
The following models are suited well for this:
- YoYoJam Sunset Trajectory NXG
I also recommend to get a string pack consisting of 5 different colors and 10 strings each.(50 strings total)
You will need so many replacement strings, because the strings might break if played for too long!
One string usually lasts 2 days for a beginner.

For teens and adults, I recommend you to get a yo-yo that requires a Bind to return to your hand. These yo-yos have the advantage that they will not "snag" (this means that the yo-yo will lose much of it's spin while moving on the string or even catch/bite the string and return to the hand prematurely) during complex string tricks and won't return to the hand in the wrong moment. This might feel weird in the beginning, as the yo-yo won't come back with just a flick of your finger, but you will get used to this quickly.
The following models are suited well for this:
- YoYoJam Journey
- YoYoJam Kickside
- YoYoFactory Velocity
I also recommend to get a string pack consisting of 5 different colors and 10 strings each.(50 strings total)
You will need so many replacement strings, because the strings might break if played for too long!
One string usually lasts 2 days for a beginner.

In this price range, I recommend Bind return yo-yos as well. You can either choose high quality hybrid (metal+plastic) or full-metal yo-yos here. Both styles have won numerous contests (including the world yo-yo contest!) so you are well equipped up to PRO status with these!
The following models are suited well for this:
- YoYoJam Dark Magic
- YoYoJam XConVict
- YoYoJam Speeder
- YoYoJam Hitman
- YoYoJam Hybrid Hitman
- Auldey L3 (Full Metal)
- Auldey Firemoon (Full Metal)
- YoYoFactory Tactic (Full Metal)
I also recommend to get a string pack consisting of 5 different colors and 10 strings each.(50 strings total)
You will need so many replacement strings, because the strings might break if played for too long!
One string usually lasts 2 days for a beginner.

I am giving the same recommdations for this price range as for the 50-Euro price limit, however, I would get a 100 pack of neon strings instead of just 50 strings, since you will need these over time anyway ;-)
Furthermore, you might want to look into cool pieces of accessoires, such as the Holster - this way, you will always have your new yo-yo with you, without damaging your pant-pockets or the stuff you carry inside them!