Need some more info to give you more useful feedback, such as what champs do you main? what role do you play most often? etc
In general, here is how building works (excluding support builds for now)
Determine your needs in lane vs the needs of your team. At the beginning of the game, you should only be concerned about YOUR needs in lane (or jung if that is the case)
Are you confident in your matchup? Build damage then, if not, build defense (HP > armor and MR at low levels)
Things to consider on your first back (Where you actually start building towards a specific purpose)
- In YOUR lane, are you ahead/even/behind?
- In the other lanes, is your team ahead/even/behind?
- are any towers down? (Team fighting about to start)
- What are the roles of your team mates vs your role? Do you have engage / disengage, are your carries able to do damage?
if you and your team as a whole are going even over all, a more generic build is fine until you can spot an obvious problem area.
Are there any champions on the enemy team that are ahead that are usually capable of solo carrying the game? In low elo that is usually champions like
if so, take a look at what items they are building and build to counter that. Are they building life steal? build a
or
Does your team lack natural CC from their kits and you need a bit more peel? consider a
or a
or even a
for the slow effects, its not as good as hard CC but if you don't have hard CC in your kit, that's the best you can really do to provide peel.
every game is its own situation and you will need to build differently for each situation, sometimes it will work out, sometimes it wont. Itemization is important but will only take you so far. My advice for climbing out of bronze, is to get good with a champion that has the potential to hyper carry.
and take advantage of the fact that at low elo, people are usually slow to group, slow to respond to split pushes, and bad at prioritizing high damage champions in team fights.