A chess rating is not granted for creating a login. Some organization has to rate your games against other rated players. Which organization matters, because the numbers are not interchangeable.
In the United States, start with a rated event
For over-the-board play in the United States, the rating schools, clubs, and class sections use is the US Chess rating. Join US Chess, enter a rated section, and play. The steps are the same for an adult and for a child whose parent enters them. This site's walkthrough of the US Chess path is the companion guide. It is aimed at that membership, not at a generic online account.
Other ratings you will see
- FIDE ratings come from FIDE-rated events. A US Chess event is not automatically FIDE rated. The conversion between the two scales is a formula, not a vibe.
- Online site ratings update after online games. They do not become a US Chess rating.
- A club handicap or a local ladder is not a published rating.
Pick the rating you will be asked for
Tournament entry forms in this country ask for a US Chess ID and a regular rating. If the question is 'how do I get a number I can put on an entry form this season,' the rated weekend is the answer. If the question is 'how do I get any number at all tonight,' an online site will give you one, and it will not be the number the entry form wants.