As a beginner, which champion is good to start with. What are the strengths and weakness for that champion and the strategy to use that champion.

Play DotA.

Monkey king.

ashe.

garen

Ashe if you're used to playing auto attacking carries.
Ryze if you're used to playing mages.
Garen if you're used to playing tanks.

dota

hm.. I would say which role you prefer.. then only decide which champ you wanna play.. pick from one of these then I will give you my recommendations

top (usually slightly tanky or semi), mid (apc), btm(adc), btm(sp), and last but not least the jungler! (depends on which type you play, usually tanky and need to help gank lanes a lot)

That really depends on your playstyle
if you like to nuke, go apc
if you like to go into action, go either top or jungle. Top if you have good mechanical skills, jungle if you have good decision making skills
the bot lane duo roles really depends on synergy with support good in zoning & map awareness, adc if you are a farm freak

if you want to play carry/adc i suggest cait or ashe both are very easy, for mid hero annie imo is the easy champ and for top you can try garen/nasus or mundo. im not suggesting your for jungle because jungle is kinda hard and if you are not familiar with jungle and you cant gank you're tend to get scold by the players so better avoid that.

Start with garen. It's the easiest.

Ashe for adc, fiddle for jg and trundle for top lane