Correct spelling, explanation: also is a word that comes from Old English, but even then it had a similar meaning to the contemporary one as it meant similarly. Grammatically, the word also is an adverb that commonly occurs both in formal and informal language.

Definition of also:
adverb – too, in addition to,
I like running from time to time and I also ride a bike at the weekends.

Idioms with also:
Some typical expressions with also are:
also-ran – a person who will probably be a loser in a competition,
He was an also-ran in the competition despite the fact that everybody counted on him.
not only… but also – when we relate two, often surprising, things,
Not only did she cheat on him, but she also stole his money.

Incorrect spelling, explanation: many English users mistakenly assume that the word also is a combination of two words – all and so – as a result, a common spelling mistake is to write also as allso. The assumption, however, is wrong as also has nothing to do with all or so, thus, do not double the -l letter in it.

Incorrect spelling

Incorrect spelling