Beginning practice sentances

I can make basic sentances such as,
ねこです, むずでわありませんでした, いぬでした。Pretty basic stuff, I don't really know where to go from here.

When you say むず, Do you mean むずい？In that case you conjugate it as むずくなかったです。

Though,  むずい is a quite informal, so if you want to use "masu" form, I would recommend you to say、むずかしい, hence -->  むずかしくなかったです。

Maybe you want to get "genki" or "Japanese for busy people"? They are pretty good for covering the beginner grammar of Japanese ^_^

頑張ってください！
がんばってください！