In every blogspot.com blog, there should be a search box in the navbar at the top left corner of the page. Putting a search term in the box will lead to a page with all the posts containing that search term.
However, some bloggers for various reasons like to hide or remove this navbar. You can replace this a Google sitesearch box like the one in the Header of this blog. However, to be able to do that, you have to have an AdSense account. If you want to apply for an AdSense account, you can click on the text link that reads "Generate revenue from your website. Google AdSense" at the top of the right sidebar. Also, this search box works in a different way from the search box in the navbar. It produces a page with links to posts that contain the search term.
There is one more way of putting a search box that gives the same result as the search box in the navbar, that is, producing a page that contains all posts rather than links. You can see an example of the search box at this blog: Good Health Information. The search box is at the bottom of the left sidebar, below the profile. Try typing in a search term (for example - diet) into the box and click "Search" and observe what is the result. You will get a page of all the posts that contain the term "diet" in it.
<div class="widget-content"><form id="searchblog" action="/search"
style="display: inline;" method="get"><input id="search-box" name="q"
size="20" type="text"/><input id="search-btn" value="Search"
If you wish, type in a title like what I have done (Search This Site), then click "Save changes". The new Page Element will appear at the top of whichever section you have added the Page Element (in my case, it was in the left sidebar). Drag and drop to whatever position you want, preview if you want, then at the top right of the Layout, click "Save".
Update 29 May 2007: In the demonstration blog Good Health Information, you may have noticed that there are 2 search boxes, one which is the Google sitesearch box right above the main column, and the one generated using the method described here at the bottom of the left sidebar. I have put the Google sitesearch bar in a more prominent position because of 3 reasons. I can chose to have the search result open in the same or different window with the Google sitesearch box. Also, the search result page opened by using the Google sitesearch box was generated by using my Google AdSense account. Further, I can configure the background color of the Google sitesearch box as described in Blending the color of your searchbox to the colors of your site. AdSense also places unobtrusive ads at the side of the search result page that are relevant to the search. I personally do not mind such ads because they are relevant to the matter at hand. In fact, I have AdSense in most of my blogs and I have to take care not to click on them even when the ad offers something that interest me, which is not infrequent. That would be an invalid click and can get me banned from AdSense. If you want a Google AdSense sitesearch box, you will have to join the AdSense program which you can do by clicking the text link that says "Generate revenue from your website. Google AdSense" at the top of the right sidebar of all my blogs, including this one.
Acknowledge: This tip was obtained from Teancum of What to do at SVU