Learn German: Das Wetter ist meistens schlecht.

In German, the verb (here: ist) must be in the second position in a normal statement. Adverbs of frequency like meistens (usually) generally come immediately after that conjugated verb.

Can I put meistens at the beginning of the sentence?

Yes! You can say Meistens ist das Wetter schlecht. But remember the golden rule of German word order: the verb must stay in the second position. So if Meistens takes up position 1, ist stays in position 2, and the subject das Wetter has to move to position 3.

Does meistens mean the same thing as oft?

Not quite. Oft means "often," whereas meistens means "usually" or "mostly." Meistens implies that it happens a majority of the time, which is a higher frequency than just oft.

Why doesn't schlecht have an ending here, like schlechtes?

Adjectives only get extra endings (like -e or -es) when they come directly before a noun, like in the phrase schlechtes Wetter (bad weather). When the adjective describes the noun but is separated from it by a verb like ist, it stays in its basic, unchanged dictionary form.