De la feria

Hoy fuimos a la feria en mi ciudad. Nos subimos a algunos juegos mecánicos. Había vendedores que vendían sus productos. Había mucha gente.

On your first sentence: The word for "Today" is "Hoy". "Fui" is an irregular verb form, monosyllable that doesn't have written accent (tilda) "í". Like many other word exceptions in Spanish, they are best learned by memory and practice. Also, since you wrote "Nosotros fuimos" on your second sentence, I assumed you also wanted to talk about "we" on your first. So it would be "Hoy fuimos" (Today we went).

I have a hard time understanding your second sentence. I think you want to say "We took some rides" as in fair attractions. For that, it would be better to use the verb "Subir" as an intransitive verb. In this case, subir means "to get on/in, to board". Paseos should be changed to "juegos mecánicos". Lit. Mechanical games/attractions. 

You need to be very careful with "to be" in Spanish. "Haber" when it refers to exist or to occur, it becomes an impersonal verb, and doesn't have a plural. It's not right to say "Habían vendedores". It''s rather "Había vendedores". The rest of the sentence would sound better as it is corrected, before it sounds like "There were vendors that they were selling their products". Please consider, "Había vendedores que vendían sus productos".

Last sentence, ""Estaban muy" means "They were very" which is a mistake since you want to "It was crowded". You could drop "allí" since the location is implicit and there is no need to specify it again.

De la feria

Today Hoy yo fui a la feria en mi ciudad. Nosotros fuimos a algunos paseos. Habian los vendedores que estaban vendiendo sus productos. Estaban muy Había mucha gente allí.

De la feria