Because a lot of Czech grammar is different from English grammar, your best start might be to learn common phrases and build your knowledge from that. Ask questions as you go, as there are a few native Czechs here.
I don't know what your motivation for learning Czech is (friends? partner? travel? study? family?), but identifying your reasons for learning would give you some direction as well.
Take the declension part (ie. changing nouns and adjectives) slowly. It's something we don't really do in English, so this would be one of the very confusing parts for you. It's almost the same system as in Slovak, and that took me ages to get my head around.
Also, take care with pronunciation, especially with the ends of words and for sneaky sounds that we don't have in English like ď, ň, ť and ř. Learn them from scratch - substitution is not possible. (Believe me, I've tried and asked.)
A lot of the challenge will just come from moving your English language habits to Czech habits. Good luck!