Hajimemashite!
I saw your sign, so I got extremely tempted to fix it... in the interest of spreading the Japanese language!
Anyways... the -masu form is used exclusively on the end of the sentence, so after 'wakarimasu' you need a full stop. Also, 'soshite' is usually used on the beginning of a sentence, so it fits nicely.
Next, instead of 'hanashimasu' (I speak), you probably wanted to say 'hanasemasu' (I can speak). Once more, a full stop should come after -masu.
When speaking, after 'Demo' you'd pause for moment, so put a comma there. 'jozu' would usually be transliterated as 'jouzu' or 'jōzu', as the 'o' sound is elongated.
'jarimasen' should be 'ja arimasen', or a more polite form 'de wa arimasen' ('ja' is informal for 'de wa').
Lastly, 'so' also has the elongated 'o' sound, same as the above-mentioned 'jouzu', and is usually transliterated to 'sou' or 'sō'.
So in the end, it would go something like this:
Ore wa nihongo ga sukoshi wakarimasu. Soshite, hanasemasu. Demo, mada jōzu de wa arimasen.
Or in Japanese writing system:
俺は日本語が少しわかります。そして、話せます。でも、まだ上手ではありません。
I hope I've been of some help, and that you'll continue to improve your Japanese ^^
And yeah, I'm quite impressed with it, since it seems you didn't study it formally. I couldn't even make a proper sentence before coming to Japan >.<
Sayounara!